U.S. Sen. Susan Collins said Monday that revelations from a forthcoming book by former national security adviser John Bolton “strengthen the case for witnesses” to be called in President Trump’s impeachment trial.

“From the beginning, I’ve said that in fairness to both parties the decision on whether or not to call witnesses should be made after both the House managers and the President’s attorneys have had the opportunity to present their cases,” Collins said in a statement.

My statement on Bolton developments. pic.twitter.com/3M59J7suts

— Sen. Susan Collins (@SenatorCollins) January 27, 2020

“I’ve always said that I was likely to vote to call witnesses, just as I did in the 1999 Clinton trial. The reports about John Bolton’s book strengthen the case for witnesses and have prompted a number of conversations among my colleagues.”

The statement follows news Sunday about an unpublished manuscript of Bolton’s book in which he says Trump tied the withholding of $400 million in military aid to Ukraine to the country’s investigation of Democrats, including former Vice President Joe Biden and his son Hunter.

Collins, who is up for re-election this fall, is among a small group of Republican senators seen as potential swing votes in the Senate’s decision on whether to call witnesses, as Democrats have demanded since before the trial started.

Collins has said a decision on witnesses should only be made after opening arguments and after senators have had a chanceto ask questions. House managers wrapped up their arguments last week and Trump’s attorneys began making their case Saturday.

Senate Democrats faced ad soft deadline last weekend to submit questions to minority leader Sen. Chuck Schumer of New York, though further questions can be submitted as the the GOP presents its defense. Republicans do not have a deadline.

As the trial got underway last week, Collins worked with a handful of Republican senators to secure language in the trial’s organizing rules calling for an up-or-down vote on whether to subpoena witnesses and documents after the questioning period.

She also played a role in pressuring Senate Majority Leader Mitch McConnell, R-Kentucky, to make changes to his proposed rules for the Senate trial, including expanding the time period for each side’s 24 hours of opening arguments to three days instead of two, and permitting House evidence against the president to be admitted automatically.

Collins voted with Republicans to table a series of amendments that called for introducing new evidence and witnesses, including Bolton, before opening arguments.

She did vote with Democrats on an amendment to give trial participants more time to file responses to motions, but that amendment, like most others, was tabled.

Collins is already facing pressure heading into the election from Democrats and independents who have targeted her seat largely because of her 2018 vote to support Brett Kavanaugh’s confirmation to the U.S. Supreme Court.

Only four Republicans votes are needed, along with all Democratic senators, to secure witness testimony. A two-thirds majority of senators would be needed to remove the president from office, which is seen as highly unlikely.

On Monday, U.S. Sen. Angus King  told MSNBC the Senate could see “perhaps double digits” of Republicans voting to call witnesses.

“I’ve been saying for more than a month that we should hear from John Bolton because he would be able to provide a first-hand account into what the President did and why he did it,” the independent from Maine said in a statement.

“At this point, I don’t see how anyone could say it’s not important to hear from Ambassador Bolton; to willfully ignore the implication that he has relevant facts undermines the idea that this is a real trial. One of the arguments put forward by the President’s defenders is that this information is all hearsay – so let’s get a first-hand source.”

Along with Collins, Sen. Mitt Romney of Utah and Sen. Lisa Murkowski of Alaska are considered Republicans who might vote to call witnesses.

(2/2) I’ve also said there is an appropriate time for us to evaluate whether we need additional information —that time is almost here. I look forward to the White House wrapping up presentation of its case.

— Sen. Lisa Murkowski (@lisamurkowski) January 27, 2020

“I stated before that I was curious as to what John Bolton might have to say,” Murkowski tweeted Monday. “From the outset, I’ve worked to ensure this trial would be fair and that members would have the opportunity to weigh in after its initial phase to determine if we need more information.

“I’ve also said there is an appropriate time for us to evaluate whether we need additional information – that time is almost here. I look forward to the White House wrapping up presentation of its case.”

Romney told reporters Monday the contents of the book make it “increasingly likely” members of his party would support hearing from Bolton.