Republican Sen. Mitt Romney of Utah said Monday that former national security adviser John Bolton's book excerpt, tying Ukraine military aid with investigations, makes it "increasingly likely" that Republican senators will vote in favor of calling witnesses during the impeachment trial of President Donald Trump, according to NBC News.

The New York Times reported on an excerpt from the unpublished book manuscript in which Bolton writes that Trump told him he wanted to withhold nearly $400 million in military security aid from Ukraine until Ukraine agreed to assist with an investigation into the Bidens — the very "quid pro quo" on which Democrats have built their impeachment case. From NBC News:

"I think, with the story that came out yesterday, it's increasingly apparent that it would be important to hear from John Bolton," Romney told reporters in brief comments. He said he hasn't fully made up his mind on calling witnesses, but what Bolton has to say is "relevant" and "therefore I'd like to hear it."

"I think it's increasingly likely that other Republicans will join those of us who think we should hear from John Bolton," Romney said. "I've spoken with others who've opined upon this as well."

Three other Republican senators are considered most likely to vote in favor of calling witnesses during the trial. Sen. Susan Collins (R-Maine) issued a statement Monday morning saying she is likely to vote for witnesses.

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

Sen. Lamar Alexander (R-Tenn.) said he will wait until after arguments are made and questions are asked to weigh in on whether witnesses should be called. Sen. Lisa Murkowski (R-Alaska), took a similar stance.

For witnesses to be called, at least four Republicans would have to join all 47 Democrats when the vote is taken on the issue later in the trial.