Just days ago, the Senate GOP appeared ready to defeat a vote to hear more witnesses sought by Democrats. But that dynamic appears shaken after it was revealed that Bolton wrote in an upcoming book that Trump told him directly that he withheld aid to Ukraine because he wanted the country to investigate his political rivals, including former Vice President Joe Biden.

Senior Republicans and Democrats alike are unsure whether the president will still get a quick acquittal with no new witnesses before week’s end. Several top Republicans tried to dismiss the Bolton bombshell on Monday, calling it "nothing new" or similar to the late-stage allegations of inappropriate sexual behavior that rocked Brett Kavanaugh's nomination for the Supreme Court in 2018 but didn't tank it.

But both Romney and Collins believe the Bolton revelation could change the contours of the trial.

“It's increasingly likely that other Republicans will join those of us who think we should hear from John Bolton. ... I have spoken with others who have opined upon this,” Romney (R-Utah) told reporters. “It’s important to be able to hear from John Bolton for us to be able to make an impartial judgment."

"The reports about John Bolton's book strengthen the case for witnesses and have prompted a number of conversations among my colleagues," Collins said in a statement. She also noted to reporters she ensured that Senate Majority Leader Mitch McConnell’s resolution setting up the framework for the impeachment trial guaranteed there would be a vote on witnesses.

But Romney's tactics drew immediate blowback from the right. Appointed Sen. Kelly Loeffler (R-Ga.), who is up for election this fall, tweeted that Romney "wants to appease the left by calling witnesses who will slander the @realDonaldTrump during their 15 minutes of fame."

Yet it was inarguable that the mood on Capitol Hill has changed drastically since the White House wrapped up two hours of its opening arguments on Saturday. Even Sen. Lindsey Graham (R-S.C.), who has been saying for weeks he doesn't want to hear from any witnesses, admitted it "could be" useful to hear from Bolton — as long as the Bidens and other GOP-requested witnesses are hauled into the Senate trial, too.

“We need to figure out what’s best for the court of impeachment, what’s best for country. And that’s something we will have to consider," Graham said. But he doused with ice water the idea of hearing only from Bolton: "If we add to the record, we need to do it completely."

"It may move the needle," Sen. Mike Braun (R-Ind.) said. "I’m not going to deny it’s going to change the decibel level and probably the intensity with which we talk about witnesses."

A spokesman for McConnell said he did not have "any advance notice" about Bolton's book manuscript.

Romney added the caveat that he would still need to hear from the White House to make a decision on whether to hear new witnesses, but he’s said all along that he wants to hear from Bolton. He also said he could not say that Bolton's testimony would determine whether he votes to acquit Trump.

Sen. Lamar Alexander (R-Tenn.), a key swing vote on witnesses, maintained his stance: He will make a decision on witnesses after the Senate's 16-hour question-and-answer period. At least four Republicans would need to join with all Democrats to advance the question of whether to seek new evidence and hear from witnesses.

So far, McConnell is prevailing. But after being down on their prospects last week, Democrats are wondering whether he could end up having to consider witnesses after all.

“It’s going to be hard to hold back. I don’t know how they do it," said Sen. Joe Manchin (D-W.Va.). “They’d be hard pressed not to have at least four or five of our Republican colleagues and friends that would” vote for witnesses.

Senate Republicans discussed the news of Bolton’s conversation with Trump, reported by the New York Times, at a party lunch on Monday. McConnell reminded senators they don't need to make a decision on witnesses until later in the week, Sen. Kevin Cramer (R-N.D.) said Monday afternoon.

As Trump attacked Bolton's credibility, some top Senate Republicans used a variety of arguments to downplay the new information from Bolton. Party leaders have been relentlessly arguing that hearing new witnesses could tie up the Senate for weeks if and when Trump exerts executive privilege to block their testimony. Executive privilege generally extends to current staffers, unlike Bolton who Trump fired in September.