Sen. Mitt Romney (R-Utah) said Monday that it is "increasingly likely" that he and at least three other Senate Republicans will vote to call former national security adviser John Bolton as a witness in President Trump's impeachment trial.

Why it matters: His comments come after the New York Times reported that Bolton alleges in his forthcoming book that Trump told him he needed aid withheld from Ukraine until it helped with investigations into Democrats, including the Bidens.

GOP sources told Axios that the revelations from Bolton's book could be enough to sway the four Republican senators needed to vote for witness testimony in the trial.

Sen. Susan Collins (R-Maine), another key swing vote on witnesses, also said in a statement that the reports about Bolton's book "[strengthened] the case for witnesses" and "prompted a number of conversations" among senators.

What he's saying:

"It's increasingly apparent that it would be important to hear from John Bolton. I of course will make a final decision on witnesses after we've heard from not only the prosecution but also the defense, but I think at this stage it's pretty fair to say that John Bolton has relevant testimony. ... I think it's increasingly likely that other Republicans will join those of us who think we should hear from John Bolton."

The big picture: Surveys released last week show that a majority of Americans would like to see testimony from new witnesses in the trial, Axios' Marisa Fernandez reports.

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