Trump had apparently told Bolton he wanted to continue holding nearly $400 million in military aid to Ukraine until officials there started investigating former Vice President Joe Biden, according to an unpublished version of Bolton’s book "The Room Where It Happened" reported by the New York Times on Sunday.
Sen. Mitt Romney of Utah said Monday it's "important" senators hear Bolton's account to make an "impartial judgment."
“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,” Romney said.
Sen. Susan Collins of Maine, another GOP moderate, said Monday she has always been "likely" to support testimony from new witnesses at the trial.
"The reports about John Bolton's book strengthen the case for witnesses and have prompted a number of conversations among my colleagues," Collins said in a statement.
Sen. Lisa Murkowski of Alaska told ABC News' Mariam Khan and other reporters: “I’ve said before I’m curious about what Ambassador Bolton might have to say. I’m still curious.”
But another GOP senator Democrats hope to win over was noncommittal.
Asked if he wants to hear Bolton, Sen. Lamar Alexander said: “I worked with my colleagues to make sure we have a chance after we've heard the arguments. After we've asked our questions to decide if we need additional evidence and I'll decide that at that time.
And at their daily pre-trial lunch, Senate Majority Leader Mitch McConnell told his colleagues to "stay the course," according to GOP Sens. Mike Rounds and Kevin Cramer.
Both men said McConnell offered repeated assurances that their game plan is sound and a vote on witnesses will happen at the appropriate time.
Cramer said it was a “keep your powder dry” pep talk.
“It’s more of a wrinkle” he added of the Bolton news.
“The whole message is still we made the right choice in the first place. We said we finished through phase one we've heard the attacks and now let’s hear the defense and then we'll ask our questions and then we make a decision on material witnesses,” said Sen. Rounds.
The Bolton news called into question what Trump's legal team knew about the Bolton manuscript and when as they argued his case before the Senate on Saturday.
In attempting to refute the Democrats’ charge that Trump intentionally withheld security assistance in exchange for an investigation, the president’s lawyer said Saturday there was “no evidence” of a connection.
“Most of the Democrats’ witnesses have never spoken to the president at all. Let alone about Ukraine security assistance,” White House deputy counsel Mike Purpura said.
Key Republicans remained unconvinced of the need for additional witnesses as recently as Sunday with some arguing House Democrats should have acquired all necessary evidence and testimony before the trial.
“If we seek witnesses, then we’re going to throw the country into chaos,” Graham.
But asked Monday whether Bolton should now be called to testify, the South Carolina senator appeared to soften his stance, not shutting the door on Bolton testifying.
“If there's a need to add to the record, then my view is that we're going to completely add to the record not selectively, and I'll let you know Thursday if I think there's a need," Graham said. "If the Senate needs to secure testimony from John Bolton, Then I will say so. If I think that's necessary for fairness, but I also have said for weeks that if we call one witness we're gone call witnesses requested by the president,"
“If we’re going to add to the record, then we’re going to go to Hunter Biden, Joe Biden and all these people,” Graham told reporters.
Graham, who has previously praised Bolton, was asked if he trusted him. Graham shot back: “I don’t know if I trust anybody right now.”
After Senate Democrats specifically called for Bolton to testify at the start of the impeachment trial, all GOP senators voted against their proposed amendments to the trial rules, including moderate Republicans who Democrats now hope will support the additional evidence.
Sen. John Thune, a member of GOP leadership, said the Bolton news is a “surprising” development for many of his colleagues and that most are “keeping their powder dry” at this point.
As for whether he has any reason to doubt Bolton, Thune says he’s withholding judgment until he sees more.
“I’m not going to make a commitment about something I don’t know about.”
Other Republicans, left their lunch, mostly ignored shouted questions from reporters about the Bolton reports.
GOP Sen. John Kennedy of Louisiana told reporters: “I’m not surprised that we’re having these leaks, these last minute allegations.”
“I think they would have more credibility if the allegations came from someone else,” Kennedy added.
“I think everybody ought to pop a Zoloft and take their meds,” Kennedy said, before adding that he thinks arguments should be finished first before the Senate considers next steps.