“In this case, it may move the needle in one direction or another,” Sen. Mike Braun (R-Ind.) said. “I am not going to deny it’s going to change the decibel level and probably the intensity at which we go about talking about witnesses."

It’s a moment that is testing GOP unity at a pivotal point in the trial. Senators are expecting to vote on whether to permit new witnesses and evidence late this week, and until the Bolton news dropped, it appeared that motion was heading for defeat. But the news has already bred hostility among some Republican.

Kelly Loeffler, Georgia’s newly appointed GOP senator, slammed Mitt Romney of Utah — who expressed interest in calling Bolton as a witness — saying he “wants to appease the left by calling witnesses who will slander the @realDonaldTrump.”

Senate Majority Leader Mitch McConnell has put a premium on keeping his caucus united amid an often unpredictable political environment and a pressure campaign from Democrats who say the trial must include witnesses and new documents to be fair. GOP lawmakers are likely to appear more on message after a Senate GOP caucus lunch Monday afternoon, which is certain to feature a discussion of how to handle the Bolton news.

In the meantime, though, the GOP message was scattered. A daily GOP news conference to discuss the trial, which was expected to feature Sens. Braun, John Barrasso of Wyoming, Mike Lee of Utah, Lindsey Graham of South Carolina and James Lankford of Oklahoma, was abruptly postponed Monday morning, only to be rescheduled with just Barrasso and Braun.

After days of knocking House impeachment managers for not offering “anything new” in their three-day opening arguments against Trump, some Republicans then turned around and criticized the Bolton claims because they were new. Bolton’s allegations, detailed in a draft manuscript of his forthcoming book, were first reported by The New York Times late Sunday.

“We’re going to have some new stuff coming out every day. That doesn’t really change anything,” Sen. Marsha Blackburn (R-Tenn.) said.

Barrasso, the No. 3 Senate Republican, had a similar response. “I think there’s going to be something new coming out every day,” Barrasso said told reporters. “New information, old information told in a different way, to inflame emotions and influence the outcome.”

But in the same news conference, Barrasso seemed to undercut his own argument: “To me, the facts of the case remain the same. There is nothing new here to what the House managers have been saying.”

But if Bolton’s claims are substantiated, they would deal a major blow to one of the White House's core defenses in the trial: that no witness has firsthand knowledge of Trump linking Ukraine aid to his desire for investigations of his political rivals.

Trump disputed Bolton’s account early Monday, saying it was a ploy to sell books. “I NEVER told John Bolton that the aid to Ukraine was tied to investigations into Democrats, including the Bidens,” Trump tweeted just after midnight.

Some Republicans vouched for Bolton’s credibility even while suggesting they would wait to see how the White House rebuts the new claims during their ongoing trial defense, which continues Monday and possibly Tuesday.

“He was fired from the job,” Sen. Jim Inhofe (R-Okla.) said when asked about Bolton’s credibility. “People change. It's kind of interesting: When that happened to Jeff Sessions, he didn't change.”

Sen. John Kennedy (R-La.) said he wouldn’t bet his house on Bolton telling the truth.

But their comments cut against Sens. Ron Johnson (R-Wis.) and Mike Rounds (R-S.D.), who both said they consider Bolton credible.

“My guess is John Bolton tells the truth,” Johnson said.

Other senators used the upcoming Iowa caucuses to question the timing of the news and the need for an impeachment trial at all when the presidency would be decided by voters in nine months — a contention sharply disputed by Democrats who say the allegations against Trump suggest he was attempting to corrupt the 2020 election.

“The timing is not coincidence, and I still don’t see an impeachable offense committed,” Sen. Steve Daines (R-Mont.) said. “The people of Iowa will be voting for president of the United States a week from today. The people should be deciding this issue, not Congress.”

Jesse Naranjo contributed to this report.