Today is the third and probably final day of presentations by President Trump’s defense in the U.S. Senate impeachment trial. The first two days of legal arguments by Trump’s team — on Saturday and Monday — haven’t offered anything particularly new if you’ve been following this story; Trump’s defense has largely echoed the talking points made by fellow Republicans over the last few months.

So I’m not expecting anything especially noteworthy to happen on camera on the Senate floor on Tuesday. Instead, I’ll be watching what the conversation looks like among Republicans on cable news and online, and looking for any news coming out of private discussions among elected GOP officials.

The reason: John Bolton.

The New York Times reported on Sunday night that Bolton, Trump’s former national security adviser, says in a manuscript for his forthcoming book that Trump directly told him that military aid to Ukraine was being withheld unless the Ukrainians committed to investigate the Bidens. Republicans had seemed on track to vote against allowing witness testimony in the Senate trial as soon as Friday, and then proceed towards an immediate acquittal of Trump. The Bolton news has thrown a wrench into all that.

On Monday, some Republican senators mused privately and even publicly about allowing some witnesses and sounded reluctant to end the trial without any such testimony in light of Bolton’s manuscript and potential information.

By Monday night, Fox News was blasting Bolton, who used to be a regular on-air guest at the network. Prominent Republicans were hinting on Twitter that Bolton leaked the book’s contents to damage Trump at the worst possible time. Meanwhile, another story published by the New York Times, on Monday night, said that Bolton’s book also details his concerns that the president was, “granting personal favors to the autocratic leaders of Turkey and China,” according to the paper. Bolton thought Trump was making foreign policy decisions to help his business properties. And Bolton, while in government, brought his concerns to Attorney General William Barr, according to The Times.

So Republicans will have to decide whether and how to alter their strategy in light of the Bolton news. That decision-making won’t take place at the trial, but it’s really the next big potential inflection point in this process.

Either way, the trial might get more interesting once Trump’s defense finishes. There is a 16-hour session that will likely take place on Wednesday and Thursday in which senators can ask questions of both the House impeachment managers and Trump’s defense team. Then, potentially on Friday, senators will vote on whether there should be more witnesses and additional documents provided for the trial. If they vote “no,” then it’s likely that a final vote on Trump’s removal from office would take place on Friday too.

So John Bolton is the story of the impeachment process right now. I expect Republicans will start in earnest on Tuesday trying to find ways to both ensure he doesn’t end up testifying, while also trying to avoid Republican senators in swing states having to directly vote against him testifying. Voting against witness testimony is likely to be unpopular with the public in light of the information that Bolton seems to have. Watch for Democrats to focus on Bolton, in the hopes that either he testifies, or that Republicans look really bad if they prevent it.

This is the first in a series of regular, short updates on the impeachment process.

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