Washington • Sen. Mitt Romney said Monday that it’s increasingly likely more Republicans will vote to allow witnesses in the impeachment trial of President Donald Trump after an explosive report highlighted potential testimony of former national security adviser John Bolton tying the president directly to leveraging U.S. aid to Ukraine in exchange for an investigation of political rivals.

Bolton’s assertion would undercut the president’s defense that Trump did not condition delivering the aid to Ukraine on announcements by the country of investigations of the Bidens and the 2016 election hacking that would likely help his reelection.

Romney said Monday it is “increasingly apparent” that it would be important to hear directly from Bolton and “increasingly likely” that other Republicans will want to hear from him.

“I, of course, will make a final decision on witnesses after we've heard from not only the prosecution, but also the defense,” Romney told reporters. “But I think at this stage, 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.”

Democrats have been arguing that the GOP-led Senate needs to allow witnesses to testify and to subpoena documents related to the president’s dealing with Ukraine at the center of the impeachment trial. Republicans, including Romney, shot down multiple requests on the first day of the trial to call certain witnesses and procure documents, though the first-term Utah senator said he would likely support efforts to call witnesses after the opening arguments now taking place.

Romney said he wasn’t sure which Republicans might join his support for witnesses — “I have spoken with others who’ve opined upon this as well,” Romney said — but at least one other senator signaled a similar position early Monday.

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

Romney told reporters that he wasn't sure how Bolton's testimony would “ultimately play on a final decision.”

“But it's relevant,” Romney said. “And therefore, I'd like to hear it.”

Senate Minority Leader Chuck Schumer, D-N.Y., said the reports about what Bolton might testify to should boost the pressure on Republicans to vote for allowing witnesses rather than provide a fast-tracked proceeding that didn't look at the facts.

“The eyes of America are upon the Republicans in the Senate,” Schumer said in response to a question by The Salt Lake Tribune. “What Mr. Bolton evidently wrote in his book ... increases the desire of the American people to have a fair trial with witnesses and documents.”

Schumer added that it wasn’t pressure by Democrats to call witnesses that Republicans should listen to but “pressure from what the American people want, a fair trial. ... Plain and simple.”

Romney is a key vote — and possibly an influential voice — that could bring witnesses to the Senate trial, a move that would prolong the now-truncated proceeding GOP leaders had hoped to wrap up this week.

The Washington Post reported Monday that Sen. Pat Toomey, R-Pa., is pushing for a deal wherein the Democrats would be able to get testimony from Bolton and perhaps others in exchange for testimony of Joe Biden and his son. Romney, the newspaper reported, is among senators considering the idea.

Romney, who has said he’s approaching the impeachment trial with an open mind, has been consistent about wanting Bolton and perhaps others to testify about their firsthand knowledge of the Ukraine situation.

A Salt Lake Tribune/Suffolk University poll earlier this month showed Utahns appear to agree with the senator that witnesses are needed.

About half of respondents say they somewhat or strongly support witnesses compared with about 31% who disagree with more testimony. The poll, with a margin of error of plus or minus 4.4%, was taken Jan. 18-22, before the Bolton news emerged.

At a closed-door lunch Monday, Romney pitched his GOP colleagues on why Bolton should testify, according to The Wall Street Journal, while Senate Majority Leader Mitch McConnell urged his caucus to stay together by noting that there was plenty of time before Friday’s vote on witnesses.

Sen. John Barrasso, a Wyoming Republican and chairman of the Senate GOP caucus, cautioned everyone against jumping to conclusions.

“I think there's going to be something new coming out every day, very similar to what we saw” at the confirmation trial of Supreme Court Associate Justice Brett Kavanaugh, Barrasso said. “New information; old information told in a different way to inflame emotions and influence the outcome. Let's take a breath. Let's listen to the president's lawyers today and the case that they present.”

Romney, whose comments fueled much of the debate about witnesses outside the Senate chamber Monday, took heat from at least one of his colleagues.

Sen. Kelly Loeffler, a Georgia Republican appointed to her job after the retirement of Sen. Johnny Isakson, slammed Romney in a tweet.

"After 2 weeks, it's clear that Democrats have no case for impeachment,” she wrote. “Sadly, my colleague @SenatorRomney wants to appease the left by calling witnesses who will slander the @realDonaldTrump during their 15 minutes of fame. The circus is over. It's time to move on!"

The impeachment trial began a week ago.

Republicans hold 53 seats in the Senate while Democrats and two independents control 47 seats.

It takes a majority vote in the Senate trial to allow witnesses, meaning four Republican senators would have to break from their party.

The president’s defense team will continue Tuesday to present its opening arguments to the Senate after which the trial will move into a phase in which Chief Justice John Roberts is expected to ask questions submitted in writing by senators.