ROBERT COSTA: New lawyers, new developments, and new questions.

The Senate trial begins with an oath to be impartial.

CHIEF JUSTICE JOHN ROBERTS: (From video.)

I do.

ROBERT COSTA: But partisan battles rage.

SENATE MAJORITY LEADER MITCH MCCONNELL (R-KY): (From video.)

House Democrats may have descended into pure factionalism, but the United States Senate must not.

HOUSE SPEAKER NANCY PELOSI (D-CA): (From video.)

What is at stake here is the Constitution of the United States.

This is what an impeachment is about, the president violated his oath of office.

ROBERT COSTA: As the president builds his legal team, senators clash over possible witnesses from John Bolton to perhaps even Hunter Biden, all as the 2020 election looms, next.

ANNOUNCER: This is Washington Week.

Once again, from Washington, moderator Robert Costa.

ROBERT COSTA: Good evening.

This week Democratic House managers solemnly proceeded toward the Senate chamber carrying the articles of impeachment against President Trump.

Those articles, however, are just part of the churning debate about the president's conduct.

New developments, new twists, and new dynamics now seem to emerge by the hour.

As Dan Balz wrote this week in The Washington Post, "Hours before Chief Justice John Roberts was sworn in as the presiding officer for the trial, the Government Accountability Office issued a report stating that the White House had broken the law by withholding 391 million in military aid for Ukraine that had been authorized by Congress."

The GAO report followed the release of documents given by Lev Parnas, an associate of Rudy Giuliani, to House investigators.

Parnas also gave interviews where he highlighted Giuliani's role in Ukraine and tied the president more directly to the activity.

LEV PARNAS: (From video.)

President Trump knew exactly what was going on.

He was aware of all of my movements.

I wouldn't do anything without the consent of Rudy Giuliani or the president.

ROBERT COSTA: Parnas was indicted in October on campaign finance charges and the White House has sharply questioned his credibility, but this split-screen moment captures the unrest facing senators who took an oath to render impartial justice.

Joining us tonight after long days at the Capitol, at the White House, and in the newsroom, Dan Balz, chief correspondent for The Washington Post; Vivian Salama, White House reporter for The Wall Street Journal; Julie Hirschfeld Davis, congressional editor at The New York Times; and Burgess Everett, congressional reporter for POLITICO.

Julie, when you look at what Lev Parnas has revealed, what have you learned as a reporter?

What stands out?

JULIE HIRSCHFELD DAVIS: Well, I think what stands out is that Lev Parnas' account and some of the documents that have come out as a result of this new release just add a lot more specifics and detail to storylines that are now familiar to all of us who have been covering the impeachment inquiry.

There are more details about how closely tied he was and Rudy Giuliani was to this pressure campaign and what the pressure campaign entailed, including an associate of Mr. Parnas who appears to have tracked Marie Yovanovitch, the former ambassador to Ukraine, and written documentation that Giuliani was passing himself off as working with Trump's knowledge and consent to get a meeting with the Ukrainian president and to get these investigations, and that this was all part of a much bigger scheme.

The substance of it was important, but also the fact of it was important - as you said, the timing was important because it underscored the fact that there's still a lot of detail that we don't have.

The president was successful in blocking any documents from coming out during the House phase of the inquiry and many witnesses, including Mr. Giuliani and obviously Lev Parnas and lots of White House officials, from testifying.

So it really underscores what we still don't know in the way of detail about how this pressure campaign unfolded.

ROBERT COSTA: Vivian, you're not only a White House reporter, you've been an ace national security reporter.

Is there a concern inside the administration about the spotlight Mr. Parnas has put now on Vice President Pence, Secretary of State Pompeo when it comes to the pressure campaign and the way Ambassador Yovanovitch was possibly surveilled?

VIVIAN SALAMA: It certainly doesn't help, but at the end of the day the way they see it is that it's his word against theirs and their word against the Democrats', essentially.

You have this man who's been indicted.

You know, he's under investigation right now in New York.

And so, obviously, that is something that they're using to their advantage to discredit anything that he wants to go out there and say, and essentially they're trying to distance the president from him at the same time and say, well, and the president himself has said I don't know this guy - yes, there are pictures and videos, but I take pictures and videos with thousands of people and so I don't really know this individual and there is no reason to say that he knows anything specific about my policies or anything like that.

He's a lone wolf.

But at the same time there he is making very specific allegations, you know, pointing to the vice president's knowledge of certain activities and especially this effort to discredit Ambassador Yovanovitch, and a governmentwide effort where he said everybody knew what was going on.

He asserts it.

Regardless of whether or not they're able to distance the president from this, at the end of the day he is an associate of the president's personal attorney and so one way or another there is now basically two degrees of separation between him and the president, and so already that's making them very uncomfortable.

ROBERT COSTA: Dan, I love the way you captured it in your story because it's not just about the articles going over to the Senate.

There is a GAO report.

There is Mr. Parnas speaking out.

What's the consequence of all of it?

DAN BALZ: Well, I think it only adds to the pressure when the Senate finally reaches that point about whether to call witnesses.

What we have is unfolding information.

This is not a static situation.

When you think about it, this is still a relatively young investigation; it started in September.

So there's much, much more that probably could be learned, and the pressure to learn that will ramp up as we go through this process.

I think, you know, they only need four Republican senators to call witnesses, and when they get to that point, based on what we've seen so far and what may come out - who knows - over the next couple of weeks, they're going to be under great pressure.

Otherwise, the Democrats are going to be able to say they did not want a real trial.

ROBERT COSTA: Burgess, no one works harder at the U.S. Senate.

So glad you're here at the table - BURGESS EVERETT: Thanks.

ROBERT COSTA: - after a long week.

When you're talking to senators on both sides of the aisle about Lev Parnas, about this new interview and new information coming out, how are they grappling with it all?

BURGESS EVERETT: I think they're treating Lev Parnas very carefully, especially the Democrats.

They've really stuck to this message devised by the minority leader, Chuck Schumer: here are the four witnesses we want - the two topline ones are Mick Mulvaney and John Bolton - and here are the documents we want.

We want more documents about the decision-making on the withholding of the Ukraine aid.

That didn't really change in the wake of the Parnas interviews.

However, you talk to folks like Chris Van Hollen of Maryland, who asked for the GAO report; you talk to Doug Jones from Alabama, he's up in a tough Senate race; and he says maybe we'll decide to call Lev Parnas as a witness in the trial.

And so you could see these things brought onto the Senate floor and have an up-or-down vote if, as Dan said, this moment comes and it succeeds and you have 51 senators that say, yeah, we should have a debate about witnesses.

ROBERT COSTA: And it's going to be not just a debate among Democrats about how to proceed; they're going to have to counter what the White House is doing and President Trump's doing.

We had new developments on Friday: President Trump added some high-profile lawyers to his legal team, including Harvard Law Professor Alan Dershowitz and former Independent Counsel Ken Starr, among others.

In an interview with the Post, Dershowitz said that, quote, "obstruction of Congress and abuse of power, the two charges that the House has brought against Trump, do not amount to high crimes and misdemeanors."

He added that he was joining the team to defend the integrity of the Constitution and to prevent the creation of a dangerous constitutional precedent.

We're seeing faces from decades past reemerge on this Trump legal team.

Dershowitz has walked a bit away from saying he's a full-fledged member of the team late Friday, but why is the White House doing this?

Do they want a TV trial?

I thought Senate Leader McConnell, based on my reporting, wanted to have a more low-key affair?

JULIE HIRSCHFELD DAVIS: Well, it's fascinating that, you know, this is sort of back to the future with the last impeachment.

You have Ken Starr.

You have Robert Ray.

You have all these sort of personalities from the 1990s and that investigation, which brings a lot of baggage that I think a lot of Republicans on Capitol Hill who were there back then do not want to relive, frankly.

That didn't work out very well for them.

But I think there is a divide, and this just sort of amplifies it, between what Mitch McConnell wants in this trial - which I think is a low-key, pretty quick affair that looks fair but in the end sort of steadily moves along to an acquittal in short order - and what the president has wanted from the very beginning, which is a theatrical trial that - where people come and vigorously defend him from what he considers to be unfair charges and an illegitimate inquiry, and he watches a lot of Fox News.

And he has seen Ken Starr on Fox News talking about the impeachment.

He has seen Alan Dershowitz for years on Fox News defending him, making legal arguments in more recent months about why these actions do not rise to the level of impeachable offenses.

So I think he really wants to see that.

And in the end, he gets to decide who his team is.

It will be interesting to see how that plays out on the backdrop of what Mitch McConnell is planning.

ROBERT COSTA: Do we know about the argument that's going to be made on the president's behalf, or is it still being developed this weekend?

VIVIAN SALAMA: It's very much being developed.

And so we're expecting that they're going to essentially try to submit some sort of case on Monday.

We're going to hopefully get more information.

Obviously now the team is coming together and, you know, those developments only in the last couple of hours.

And so with that we expect by Monday, at least, that we will have more clarity as far as the direction that the White House wants to take this.

ROBERT COSTA: Dan, what's the challenge for House Democrats?

The managers were appointed this week by Speaker Pelosi.

They're now going up against Alan Dershowitz, Ken Starr.

What do you make of how Speaker Pelosi is handling this, and how she's going to present her case with her managers?

DAN BALZ: Well, I think we're going to see very much what we saw in the House in terms of the presentation of the case.

There was some information late this afternoon that came off the Hill that indicated that they're going to try to put into the record some of the new information that has come out.

Now, the Senate Republicans are basically saying: No new information.

You have to bring what was in the investigation in the House.

So there's going to probably be some clashes early on as the presentation is made.

And we'll see about that.

But I think they see this as one more opportunity to drive home what isn't known, who hasn't been able to be heard from yet, and to keep pushing and pressuring on that front to put the Republicans in a difficult position.

But they're in their own difficult position, because we know the country is so badly split on this that there's a fine line that they have to walk.

How much do they want to push this?

How far do they want to push it?

And to what extent it puts some of their own people - ROBERT COSTA: To that point, perhaps it's not just about the case being made in the Senate chamber, but about the court of public opinion.

DAN BALZ: Well, very much so.

I mean, you can see from what the president is doing, by adding the people he's added to this, that he does see this as, you know, not simply something that's playing out in the Senate chamber.

But this is something of a national discussion and debate.

ROBERT COSTA: Let's get a little bit more into the whole standoff over witnesses, because Lev Parnas, he's far from the only possible witness on the Senate's radar.

And this week, that standoff over witnesses became more politically charged, as Burgess scooped for POLITICO.

Senate Majority Leader McConnell quietly invited Republican senators, many conservatives, to his office to discuss strategy.

And during that 30-minute session, Senator Ted Cruz pitched McConnell on the idea of witness reciprocity.

If Democrats want to hear from John Bolton, the former national security adviser, then Republicans get to hear from Hunter Biden.

Could Democrats call the Republicans' bluff here and say: We'll take that deal.

Hunter Biden for John Bolton.

What do you see playing out, Burgess?

BURGESS EVERETT: I feel like right now there are not the votes to bring Hunter Biden in front of the Senate.

That's not just from the Democratic side, where I think you're really looking at a very small universe of Democrats who would really be wanting to have this sort of circus-like atmosphere, and the witness argument essentially just sort of blow up in their face, I think is how that would be seen.

There's also some Republicans, you know, that are not necessarily eager to do this either.

I still go back to this moment a few weeks ago where Susan Collins was talking to reporters and asked: What sort of witnesses do you hear from?

This was at the moment where she was saying, I'm a juror, I don't really want to talk about this.

But she did say something, which was I want to hear from relevant witnesses.

And if you sort of read between the lines on that, that's sort of throwing cold water on the idea of bringing someone like Hunter Biden in, even though he's tangentially related to the impeachment case.

It's hard to say he's directly implicated.

ROBERT COSTA: But who has power here, Burgess?

Is it McConnell or Collins?

Is it the moderate wing?

Is it the conservatives, like Cruz, who, along with the White House, are taking a hardline?

Where is the power center?

BURGESS EVERETT: The power center is McConnell, for sure.

But how does he convince the three to four senators that he needs to get to go along with him to shut down the witness votes?

ROBERT COSTA: Well, you have Senator Schumer trying to create pressure points with possible votes and difficult votes on the floor about witnesses.

JULIE HIRSCHFELD DAVIS: Right.

And I mean, I - Mitch McConnell is always the power center in the Senate.

But I do think, and he's actually said publicly, that he doesn't have all the ball control in this situation that he normally does on the Senate floor.

And that is because some of the - all of these questions are going - the procedural questions are going to be decided on a 51, a simple majority, vote margin.

And that means that the four senators kind of in the middle here - Susan Collins, Lisa Murkowski, Lamar Alexander, Mitt Romney - are very important.

And so if they're not going to go along with a Hunter Biden, for instance, then that's not going to happen.

If they're not going to go along with, for instance, a quick dismissal, which a lot of conservatives want to see right off the bat when the trial starts - ROBERT COSTA: What about Schumer?

JULIE HIRSCHFELD DAVIS: Schumer is trying to create pressure on Republicans, on Susan Collins, on the other Republicans who are up for reelection in 2020.

And he believes that this debate is going to do that, and that they're going to be hard-pressed to defend a vote against hearing from more witnesses given that new information is coming out and that people have been blocked from testifying.

VIVIAN SALAMA: A lot of it could be also saving face too, is that some of these Republicans perhaps don't think that bringing Hunter Biden in is actually, like, a reasonable solution, but at least the president has been demanding that he wants to see Hunter Biden come in.

At least they show that they're making some sort of effort to do that and appease him as well.

And so there's a little bit of that too, is sort of posturing to the White House's demands - the president specifically.

BURGESS EVERETT: And what's now going to happen is Chuck Schumer's just - he's not going to get his four witnesses and the documents only.

That is just not going to happen.

The idea of witness reciprocity, Senator Collins does not hate that idea.

She says, no, we're not going to just bring in the Democratic witnesses.

The president's counsel should have a chance to bring in witnesses too.

And so that - you can see how what Schumer's really done right now is he's driven a wedge in between the two parties.

And so when all the Republicans vote no next week on his witness package, he's going to see some political benefit to that.

ROBERT COSTA: What does this mean for the outlook in 2020 for a lot of these senators who are up for reelection?

DAN BALZ: I think there's in a difficult position.

The public will be watching this to judge whether it's fair.

We know that there are unanswered questions.

And if it looks as though people are in a hurry to get this out of the way, in other words to sweep it under the rug, I think some of the - some of the senators in - you know, in these difficult races, the Republican senators, could pay a price for that.

But if you're Susan Collins, for example, almost whichever way you go on this is going to have consequences.

ROBERT COSTA: Like the Kavanaugh vote in a way.

DAN BALZ: Right.

And so I think that, you know, for those senators there's going to be some agony on some of these tough questions.

ROBERT COSTA: That point about dismissal - when I was at the Senate this week, I kept hearing from conservative senators privately, there's going to be a lot of pressure from the White House to dismiss this as soon as possible.

So even as witness negotiations are ongoing, you could have President Trump tweeting, making speeches at rallies saying: End this trial.

VIVIAN SALAMA: Oh, yeah.

I mean, everybody wants this to be quick I think - ROBERT COSTA: At the White House.

VIVIAN SALAMA: At the White House, and also a number of lawmakers as well, who are - the senators who are on there.

You have the Democrats who want to get back onto the campaign trail - a number of them - and Republicans.

Mitch McConnell has said that he wants a quick - a quick trial as well.

The White House definitely wants a quick trial, but also the president wants to quickly be able to go out there and feel vindicated and say, well, you know what?

This was a hoax from the beginning.

This was a witch hunt.

And go to his - you know, his constituents, and essentially use it as a tool to get him reelected.

ROBERT COSTA: But do they want a dismissal, or do they want - does he actually want Hunter Biden on the stand, President Trump, based on your reporting?

Does he want that spectacle, or would he rather just have the whole thing dismissed quickly?

VIVIAN SALAMA: He definitely enjoys a spectacle.

And that's something that he's made clear.

And so I think that he would possibly enjoy it.

But there's a risk to be taken.

And obviously a lot of his advisors at the White House do know that.

ROBERT COSTA: Vivian brought up how many Democratic senators want to get back on the campaign trail.

And that's true, because several Democratic presidential candidates are going to be stuck in their chairs in the Senate, just weeks before the Iowa caucuses.

But there were fireworks this week in Iowa at the CNN debate.

And two of the liberal wing's leading contenders - Senator Bernie Sanders and Senator Elizabeth Warren - had a tense exchange.

At issue, a 2018 private conversation between the two senators.

Warren has said that Sanders told her he did not think a woman could beat President Trump.

Sanders has forcefully denied saying that.

And after the debate, Warren did not shake Sanders' hand.

Here is that back and forth caught by the microphones.

SENATOR ELIZABETH WARREN (D-MA): (From video.)

I think you called me a liar on national TV.

SENATOR BERNIE SANDERS (I-VT): (From video.)

You think I did what?

SENATOR ELIZABETH WARREN (D-MA): (From video.)

I think you called me a liar on national TV.

SENATOR BERNIE SANDERS (I-VT): (From video.)

You know, let's not do it right now.

You want to have that discussion?

We'll have that discussion.

SENATOR ELIZABETH WARREN (D-MA): (From video.)

Anytime.

ROBERT COSTA: Julie, what does this suddenly divided left flank in the Democratic Party mean for the 2020 contest?

JULIE HIRSCHFELD DAVIS: I mean, I think what it does is just bring kind of out into the open what has been a simmering or, depending on how you want to look at it, very chilly sort of dynamic between the two of them for some time.

I mean, they are competing in many ways for the same slice of the Democratic primary electorate.

And we're getting now toward the voting contest where the primaries get underway in earnest.

And they're really grappling.

And I do think that the way that they behave and the way that this exchange played out is going to affect people's, voters', you know, views of them, whether that - you know, for good or for ill. And it is also fascinating that they now are going to be in the Senate chamber for hours on end, just feet away from each other, and having to not only miss their campaign face time but also deal with each other in very close proximity, and that will play out also probably in the halls of the Capitol in the next few weeks.

BURGESS EVERETT: I was definitely looking yesterday when they were getting sworn in to see - JULIE HIRSCHFELD DAVIS: Yeah, we're all looking for the body language.

BURGESS EVERETT: - what's the body language going on there.

ROBERT COSTA: What was the body language?

BURGESS EVERETT: I couldn't see anything, and it seemed - ROBERT COSTA: You could see a few things.

BURGESS EVERETT: I mean, I couldn't see any body language.

They were stiff.

They weren't looking at each other.

There was - ROBERT COSTA: They're impartial jurors.

(Laughter.)

JULIE HIRSCHFELD DAVIS: Well, Senator Sanders was asked I think it was today, have you spoken to Senator Warren since then, and he said nope and he kind of walked away from the camera, and so - (laughs).

BURGESS EVERETT: That's better than I got the day before, which was I don't want to talk about - just no answer whatsoever.

And I think Elizabeth Warren's also said this is the last I've said of this, so you can see how they are sort of - I think they both realized this moment didn't look great for either of them.

ROBERT COSTA: Dan, what's your thought on this moment, the Sanders-Warren interaction?

This debate, 2020, seems to for months have been about Medicare for All, debates over healthcare; now it's intensely personal.

DAN BALZ: It is, and I totally agree with Julie that this was - this was a collision destined to happen.

They're basically fishing in the same pond for a lot of - a lot of voters; not entirely, but for a lot of voters.

They share many of the same views of what's wrong with the system.

They have some different prescriptions on how to fix it, but they have a similar worldview about a rigged system or the influence of money.

And it's not likely that the finalists for the Democratic nomination - I won't say that definitely, but - are going to be the two most liberal candidates in the race.

And so in one way or another, both in Iowa and New Hampshire they are competing very much against each other.

ROBERT COSTA: So is Vice President Biden, Mayor Buttigieg, Mayor Bloomberg, are they all watching and saying this is creating a little bit more space for us as the left fights the left?

DAN BALZ: I think that's right, although, you know, because Iowa is so fluid right now, any one of four candidates seems to be in a position to either win Iowa or finish fourth, and the consequences of either of those is significant for all of them.

So I think that the more that Biden can stay out of the line of fire and let the story for a while be Sanders versus Warren, he's quite happy with that.

But ultimately they've all got, you know, two-and-a-half weeks to make the final sale, and some of them are stuck here and some of them are going to be out in Iowa much of that time.

ROBERT COSTA: How will impeachment affect Vice President Biden's campaign?

VIVIAN SALAMA: I mean, he's obviously taken questions but also kind of struggled to try to get the message back on track to his - you know, his platform, essentially.

But obviously, this is hovering around him.

He has a bit of a legacy that he comes to the table with in terms of his, you know, being vice president to President Obama, who was very popular, remains very popular in the Democratic Party, and so he's hoping to, you know, use that to his advantage.

But given the way they look going into Iowa, it's hard to say if it's what's going on with his son and the whole controversy with impeachment or if it's just other things.

ROBERT COSTA: Talking about using things to your advantage, President Trump took a lot of action on trade this week.

The USMCA was passed by the Senate.

He signed phase-one trade deal with China.

Inside the White House, are they trying to build a story outside of the impeachment narrative?

VIVIAN SALAMA: Oh, of course, and this is something that they desperately want to focus on because they consider it big wins for him going into the 2020 campaign.

This is something that he has been trying to get across for ages.

It was a very contentious issue with Congress in terms of trying to get the USMCA passed in particular.

And so you had that in the last couple of days, plus he managed to sign phase one of the China trade deal which the White House argues is going to benefit farmers, especially in critical Trump states, and - critical districts that could vote for Trump.

And so it's going to be interesting to see, but certainly that is where they would love all the attention to be right now.

JULIE HIRSCHFELD DAVIS: You hear President Trump and see him tweeting all the time about how can you impeach a president when the economy is so good, and I think this week's trade accomplishments really sort of fed that narrative for him, and I think they're really - he's really hoping to continue coming back to that.

Of course, the one is not related to the other, but I think that he thinks that it's possible to kind of shift the focus to something else and is trying to compartmentalize in some ways, although we don't see him doing that in the same way Bill Clinton did when he was being impeached.

He talks about impeachment and seems obsessed with impeachment all the time, but obviously is quite happy whenever he can point to anything that, you know, is a - is a concrete accomplishment.

BURGESS EVERETT: The timing of the trade vote was kind of interesting in the Senate.

ROBERT COSTA: Why?

BURGESS EVERETT: The fact that it came right before the impeachment inquiry seems notable to me, and it's because of Speaker Pelosi's idea to hold onto the articles a little bit longer than folks had hoped, and you could easily see a world where USMCA has to wait until impeachment is over.

That gives the Republicans a decent talking point to say we need to finish this up and get on to do the people's business, USMCA's just waiting for - waiting for us and we need to move this more quickly.

That backstop isn't there anymore, so it has sort of taken away this sort of talking point about Democrats are so obsessed with impeachment we can't do anything.

You could argue over the past couple months, at least in the Capitol, that Washington's been working way better than it has been for a couple years.

ROBERT COSTA: Dan, USMCA, China trade deal, do they fade as issues or is this part of the president's reelection strategy, at the core of his whole argument here?

DAN BALZ: Well, it's certainly part of his reelection strategy.

The problem is the president often gets in the way of his own strategy, and I think that's the danger.

ROBERT COSTA: That's all we have for tonight.

We'll talk about this more, but be sure to watch PBS's coverage of the Senate impeachment trial with Judy Woodruff and the PBS NewsHour team starting next Tuesday at 12:30 p.m. Eastern.

And check out our Washington Week Extra.

As I told Dan, we will continue to discuss the politics of trade in 2020 and the outlook in Iowa.

It's available as a free podcast or you can watch it on our website.

I'm Robert Costa.

Good night.