(CNN) — Republican Sen. Mitt Romney of Utah predicted it was “increasingly likely” that others would join him in calling for Bolton to testify, and GOP sources say the revelations add new uncertainty to this week’s expected witness vote after Republicans were confident it would be defeated when the Senate gaveled out on Saturday.

“I can’t begin to tell you how John Bolton’s testimony would ultimately play on a final decision but it’s relevant,” Romney told reporters Monday. “And therefore, I’d like to hear it.”

President Donald Trump’s former national security adviser has upended the Senate impeachment trial, and new revelations from John Bolton’s draft book manuscript could turn the tide on whether senators call for witnesses.

Since the Bolton news broke, the White House has heard from Republican senators frustrated that they were kept in the dark when at least someone in the White House had the Bolton manuscript since the end of December, according to a source familiar with the conversations.

A source with direct knowledge of the manuscript told CNN the Times’ telling of Bolton’s account of the Ukraine aid hold discussion with Trump is accurate.

The President’s legal team resumes its second day of arguments at 1 p.m. ET Monday, but all of the attention will be focused on the Republican senators sitting in the chamber and how they react Sunday night’s New York Times bombshell that Bolton’s draft manuscript says Trump told him US security assistance to Ukraine was conditioned on investigations into Democrats, including former Vice President Joe Biden.

Democrats seized on the allegations described in the draft manuscript to demand Bolton’s testimony in the Senate, saying anyone seeking to learn the truth should want to hear from the President’s former national security adviser in light of the revelations.

“It completely blasts another hole in the President’s defense,” House Intelligence Chairman Adam Schiff said on CNN’s “New Day” Monday morning. “I don’t know how you can explain that you wanted a search for the truth in this trial and say you don’t want to hear from a witness who had a direct conversation about the central allegation in the articles of impeachment.”

Bolton eyes testifying for weeks

Bolton has flirted with testifying in the impeachment trial for several weeks now, after Democrats say his lawyer threatened to file suit if he were subpoenaed to testify during the House impeachment inquiry.

Earlier this month, Bolton said he was willing to testify in the Senate if he was subpoenaed. But throughout the first week of the trial, it did not appear there would be the votes to call any witnesses, with only two Republican senators — Romney and Sen. Susan Collins of Maine — expressing public interest in additional witnesses and documents.

Some Republicans downplayed the news from Bolton. Sen. John Cornyn, a Texas Republican, said the “aid did flow and the investigation never occurred, so we really are talking about events that never happened.”

“Additional witnesses may be sought for both sides — I presume that the President’s team might want Joe Biden or Hunter Biden subpoenaed as well, and you can see how quickly this will begin to unravel and basically dominate all of the news leading up to and including the election,” Cornyn said. “We have an election in nine months, I think it’s better reserved to the voters to make the decisions at this point ”

The President’s legal team is expected to raise the Bidens in its opening argument, which the defense team will continue to deliver on Monday and potentially on Tuesday, the President’s personal attorney Jay Sekulow said last week.

On Saturday, the defense team spent two hours walking through an overview of its argument without mentioning the Bidens, seeking to poke holes in the House’s case — including that the House didn’t have any direct evidence linking the President to a quid pro quo.

What happens next

It’s unclear how — or if — the President’s legal team will address Bolton on Monday.

After the defense team concludes its arguments, there will be 16 hours for senators to ask questions of both sides. The Senate’s trial rules then call for a debate and vote on whether the Senate generally should seek more witnesses and documents.

If that vote fails, the trial is likely to conclude with an acquittal vote this week. But if it succeeds, the trial will enter into an unpredictable phase where both senators and the legal teams could propose witnesses, which the Senate could vote on.

In addition to the debate over Bolton’s testimony, Schiff argued that the Senate must also seek his notes — as House Democrats were stonewalled from receiving documents from the Trump administration during the impeachment inquiry, even from witnesses who testified before the House.

“We learned John Bolton took detailed notes and presumably these are contemporaneous. These notes took place while the events were happening, while they were fresh in his mind. Those in many respects are more important than the manuscript,” Schiff said. “We ought to not only have John Bolton testify but we ought to see what he wrote down in his notes at the time.”