Republican Senator Susan Collins of Maine supported comments made by Sen. Mitt Romney (R-UT) over whether former National Security Adviser John Bolton should testify in President Trump's impeachment trial, after a manuscript of his upcoming book was leaked to the New York Times which claims that President Trump explicitly linked a hold on Ukraine aid to an investigation of the Bidens.

"The reports about John Bolton's book strengthen the case for witnesses and have prompted a number of conversations among my colleagues," said Collins.

JUST IN: GOP Sen. Susan Collins: "The reports about John Bolton's book strengthen the case for witnesses and have prompted a number of conversations among my colleagues." https://t.co/wDglFX1ipA pic.twitter.com/DlSjXMfDsk — ABC News Politics (@ABCPolitics) January 27, 2020

Collins echoed Monday comments by Romney, who said "it is increasingly apparent that it would be important to hear from John Bolton," adding that it is "increasingly likely" that other GOP senators would join the 11th hour call.

"It's pretty fair to say that John Bolton has a relevant testimony to provide to those of us who are sitting in impartial justice," he said.

WATCH: GOP Senator Mitt Romney says "it is increasingly apparent that it would be important to hear from John Bolton," adding it is "increasingly likely" other GOP senators would join in too. pic.twitter.com/gyeiKkyPuE — Meet the Press (@MeetThePress) January 27, 2020

Update (1:00 p.m.): Surprise, surprise - Sen. Murkowski now says she's 'curious as to what Bolton might say.'

Other GOP Senators disagree - including Majority Whip John Thune, who said "I don't think it changes the facts ... I don't personally see it as a game changer."

Sen Fischer asked if the Bolton revelations change anything for the trial: “Do you guys have memos on the same question to ask all the time? Just curious.” — Erica Werner (@ericawerner) January 27, 2020

Sen. John Cornyn (R-TX) said "This looks like a marketing tactic to sell books is what it looks like to me."

Sen. Blunt on John Bolton:



“I can’t imagine that anything he would have to say would change the outcome of the final vote. Might be interesting, might be an oversight question that Congress wants to take months to pursue.“



“I think Bolton is credible, he's a friend of mine.” — Alan He (@alanhe) January 27, 2020

SHOTS FIRED



Sen Kelley Loeffler R-GA dings Sen Mitt Romney R-UT, accuses him of trying to "appease the left" by calling for witnesses who would 'slander' President Trump https://t.co/b423bJtjp4 — Jamie Dupree (@jamiedupree) January 27, 2020

President Trump, meanwhile, tweeted on Monday "I NEVER told John Bolton that the aid to Ukraine was tied to investigations into Democrats, including the Bidens. In fact, he never complained about this at the time of his very public termination. If John Bolton said this, it was only to sell a book. With that being said, the transcripts of my calls with President Zelensky are all the proof that is needed, in addition to the fact that President Zelensky & the Foreign Minister of Ukraine said there was no pressure and no problems. Additionally, I met with President Zelensky at the United Nations (Democrats said I never met) and released the military aid to Ukraine without any conditions or investigations - and far ahead of schedule. I also allowed Ukraine to purchase Javelin anti-tank missiles. My Administration has done far more than the previous Administration."

...transcripts of my calls with President Zelensky are all the proof that is needed, in addition to the fact that President Zelensky & the Foreign Minister of Ukraine said there was no pressure and no problems. Additionally, I met with President Zelensky at the United Nations... — Donald J. Trump (@realDonaldTrump) January 27, 2020

Trump: "Nothing was ever said to John Bolton." lol pic.twitter.com/7Ht4jA55EA — Josh Marshall (@joshtpm) January 27, 2020

Of course, if Bolton doesn't testify, they'll call it a cover up.