Welcome to FiveThirtyEight’s weekly politics chat. The transcript below has been lightly edited.

sarahf (Sarah Frostenson, politics editor): On Sunday, The New York Times reported a bombshell from former national security adviser John Bolton’s unpublished manuscript about his time in the White House: President Trump had, in fact, wanted to withhold $391 million in assistance to Ukraine until investigations into Democrats — including Joe Biden and his son Hunter — were agreed to.

Bolton’s version of events undermined the president’s claim, which has been an important part of his defense during the impeachment process, that the decision to freeze aid to Ukraine was independent from requests that Ukraine investigate the Bidens. The reported contents of the manuscript also gave a snapshot of the kind of testimony Bolton might give if the Senate allows witnesses to be called in the trial.

The White House legal team is back in the Senate today presenting the president’s side of the case, but what should we make of Bolton’s manuscript? Does it change the calculus of the trial moving forward?

ameliatd (Amelia Thomson-DeVeaux, senior writer): Well, it certainly amps up the pressure on GOP senators to call witnesses. Bolton has been the white whale of the impeachment inquiry since last fall, since he seems to have been privy to a lot of key conversations around the Ukraine aid and the investigations. The NYT reporting just underscores how important his testimony could be.

perry (Perry Bacon Jr., senior writer): My impression is that Bolton wants this known — he is willing to back up what others have testified to: The aid was tied to launching investigations. I didn’t find this too shocking — there is ample evidence aside from Bolton’s words that the Ukrainians were being asked to investigate the Bidens in exchange for the aid. But Bolton was in the White House and “has the receipts,” as the kids say.

Reactions from Senate Republicans so far also suggest that Bolton’s version of events might have some impact. The senators who were already generally supportive of having witnesses at the impeachment trial (Susan Collins and Mitt Romney in particular) are now sounding firmer in that stance. And other Republicans are also now suggesting that they could be open to calling witnesses, although they are talking about summoning Hunter and Joe Biden as well as Bolton.

ameliatd: Some GOP senators have responded defensively to the Bolton news, and that could be important, too. One critique is that he’s just trying to juice books sales; another is that he doesn’t actually need a subpoena to share what he knows (which is true!).

The thing is, if Bolton wants this information to be known, he can make it known at any time by holding a press conference. But he seems to want permission from his party to share the info, in the form of a subpoena.

sarahf: So as Perry says, we didn’t really learn anything new here in Bolton’s manuscript. Rather, Bolton’s account essentially corroborates what others from the Trump administration told House investigators last November. But unlike those other witnesses, including the former top U.S. diplomat in Ukraine, William Taylor, and American E.U. Ambassador Gordon Sondland, Bolton was “in the room.”

Is that, in and of itself, a game changer? I guess at the very least, it probably does shift the vote count on the motion to call new witnesses. A few days ago, it didn’t seem as if Democrats were going to win over enough GOP senators to make that happen.

ameliatd: It’s an interesting question, Sarah, because Bolton is different from the witnesses we’ve heard from so far in a couple key ways: 1) He’s a political appointee (almost all of the people we heard from in November were career officials); 2) He’s a longtime Republican hawk who should have significant cred if he’s saying that what Trump did was out of bounds.

On the other hand, Bolton left the Trump administration on pretty bad terms with the president, who is denying he told Bolton that the aid was tied to the investigations.

It does feel like if this doesn’t shift the vote count for calling new witnesses, nothing will.

perry: One of the big GOP talking points has been that this is all hearsay. Except now Bolton is suggesting he heard directly from Trump that the goal was to force an investigation of the Bidens or Ukraine would not get aid. I expect Republicans will eventually just pick a new talking point, though — “executive privilege” or “Bolton was disgruntled” or something.

ameliatd: And that’s why I think it’s telling, Perry, that GOP senators are already saying this is just a play to sell copies of his book.

perry: Bolton testifying would probably be really bad for Trump — so there is very little incentive for Trump and the Republicans to have him testify.

sarahf: Right, and Trump definitely seems to be busy already sowing seeds for a defense that Bolton is disgruntled/untrustworthy, saying that he never discussed this with Bolton.

perry: As always, the key players here on whether we hear from new witnesses are the Republicans from swing states: Cory Gardner of Colorado, Martha McSally of Arizona and Collins, along with Trump-skeptical senators like Lisa Murkowski of Alaska and Romney. In my view, they need to find a way to placate Trump and his base but also their state’s more centrist voters. I don’t know what that looks like.

ameliatd: Yeah, and even though Republican support for calling new witnesses has gone down since the end of December — according to our polling with Ipsos, using Ipsos’s KnowledgePanel — 41 percent of Republicans still want new witnesses. (Of course, they could have witnesses other than Bolton in mind, like the Bidens.)

I guess my somewhat cynical take on this is similar to Perry’s — except for the people who were already apt to vote for witnesses, Republicans might just find other reasons to avoid calling Bolton. And frankly, I think the way Bolton has handled this makes that easier for them to do.

sarahf: So Amelia, you don’t think this tips the scale in terms of calling new witnesses?

ameliatd: I mean, I think it’s hard to predict right now. It’s totally possible that the balance does tip and we see more senators voting for witnesses. But GOP senators are already questioning Bolton’s motivations from a number of angles, and it’s not like their defenses of Trump have been especially consistent or logically cohesive to this point.

sarahf: My thought is that this is about as bad as news can get for Trump. This isn’t someone with secondhand knowledge alleging he withheld aid for politically motivated reasons. It’s a close former adviser with firsthand knowledge of these conversations. I think this now forces Republicans’ hand and they’ll have to call him.

Although, to be clear, I think there are still ways Republicans can spin this so it’s not a total defeat. Take Sen. Lindsay Graham. He’s now saying “all relevant witnesses” should be called, including ones Trump has requested, which I read as code for Biden.

perry: One of the unknowns is here whether Democrats will agree to one of the Bidens testifying if Bolton and others who will make Trump look bad are asked to testify.

ameliatd: Right, I don’t see a world where we only get Bolton, if witnesses are called. Some GOP senators have been pushing this “witness reciprocity” (Bolton-for-Bidens) line for a while, so it’s not surprising to see that resurfacing.

perry: What a move by Bolton. He is now going to join the likes of James Comey, Robert Mueller, Jeff Flake and others who are fairly conservative and were once in pretty good standing with the Republican Party, but then criticized Trump and were condemned by their former allies. Bolton was a fixture on Fox News before he joined the administration. Those days are probably over. He will be cast as part of “The Resistance” now.

And this will be another illustration of where today’s conservative moment is headed: You either support whatever Trump does or you aren’t going to remain in good standing at major conservative power centers , like Capitol Hill and Fox News.

We should note two other things: 1. The recording that came out last week in which Trump seems to be ordering the firing of Marie Yovanovitch, who was then the U.S. ambassador to Ukraine, and 2. Secretary of State Mike Pompeo getting into a tense dispute with an NPR reporter who asked him about Yovanovitch and the circumstances of her recall from Ukraine. There are a lot of people tied to Trump involved in this Ukraine controversy who are not doing a particularly good job defending Trump, or, in Bolton’s case, not defending him at all.

The Democrats can’t keep this trial going forever. But it does look like evidence is going to keep coming out that makes Trump look worse, so they must be trying to figure out how to keep this trial going on beyond this week. And the Republicans have to be thinking that witnesses are a net negative for them, even if they get Hunter or Joe Biden to testify.

ameliatd: I will add also that the Republicans/Trump administration have been pushing the idea that Bolton’s testimony could be covered by executive privilege, which would snarl things up in the courts. They could certainly try that tack, but executive privilege claims are on shakier footing legally as more of this information comes out publicly. And also, if a majority of senators vote to subpoena Bolton and he wants to testify, it could be very hard for Trump to stop him.

sarahf: Tell us a little bit more about how that would work, Amelia. It seems to me that the administration could do what you’re describing, but they risk making a pretty unpopular move.

Because we’ve found in our own polling with Ipsos that a majority (59 percent) of Americans still want to hear new witnesses, and ABC News/Washington Post found in their latest poll that 66 percent of Americans favor the Senate calling new witnesses — and that was before the Bolton development. I have to imagine that number goes up.

ameliatd: Executive privilege is a pretty ill-defined power that presidents use to keep internal communications secret. In some cases, it has protected senior White House aides from responding to a subpoena. But those people were also unwilling to testify. The Trump administration could try to get a court to issue a restraining order preventing Bolton from testifying, but we’d be headed into unprecedented legal territory if that happens. And it’s not even clear that what Bolton would share would be covered under executive privilege to begin with.

It’s also possible that the courts would be less inclined to rule in the executive branch’s favor during an impeachment trial.

And then of course, there’s the fact that it looks pretty bad if Trump keeps someone with clearly relevant information from testifying. On the other hand, he’s been doing that for months and it hasn’t shifted the needle. But maybe Bolton is different, especially if he’s out in public making a ruckus about what he knows and how he wants to share it.

sarahf: Right, and I suppose the other thing here is: say Trump’s legal team did go down this path … it doesn’t stop Bolton from publishing his memoir, right? Though, I suppose the administration could push to exclude certain excerpts during the standard review process. It just seems as if from a PR point of view, we’re past that point, and so now the Trump team’s best of course action is to discredit Bolton, as they have tried to do with the other witnesses.

Assuming we do hear from Bolton because the Senate decides to call new witnesses, how could this change things? Does it cause some Republicans reconsider their vote to acquit Trump?

perry: Yeah, Bolton I think is unique in that: 1. My impression is that he doesn’t like Trump and wants this fight. 2. He served in the administration in a very senior-level job and probably knows a lot, some of which would be embarrassing for Trump if it came out. This may not matter for the impeachment trial, but if I were Trump, I would be a little worried about Bolton.

I don’t think this affects the bottom line — I don’t think Republicans will ever vote to remove Trump. (There is a very, very small but I think plausible scenario in which they privately urge him to resign.) But do I think it’s possible we have a longer trial with witnesses? Yes, and I think that possibility is much higher than it was on Friday.

But ultimately I think still it’s unlikely that we get witnesses — it’s just not in the interests of the GOP, and they probably still have the votes to prevent it. But a trial with witnesses would be a wild card — you never know what people will say.

ameliatd: Right, and Bolton is a much more well-known figure than any of the other people we’ve heard from so far. Plus, he seems like he could fill in the piece that’s been missing for Democrats all along: a witness who heard from Trump’s mouth that the aid was tied to the investigations.

Hard to see how that wouldn’t damage Trump in some way, even if he still gets acquitted.

Make sure to check out FiveThirtyEight’s Democratic primary forecast.