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A pair of moderate Republican senators said Monday that a report of major revelations in a soon-to-be-released book by John Bolton strengthens the case for calling witnesses in President Donald Trump's impeachment trial — and even a top ally of the president said Bolton would be a "relevant" witness.

Sen. Mitt Romney, R-Utah, said it's "increasingly likely" there will be enough Republican senators to vote to call witnesses.

"I think, with the story [about Bolton's book] that came out yesterday, it's increasingly apparent that it would be important to hear from John Bolton," Romney told reporters. Romney said he hasn't fully made up his mind, but he said that what Bolton has to say is "relevant" and that "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."

Trump fired Bolton as his national security adviser in September.

Another moderate Republican senator, Susan Collins of Maine, tweeted a statement saying the "reports about John Bolton's book strengthen the case for witnesses and have prompted a number of conversations among my colleagues."

My statement on Bolton developments. pic.twitter.com/3M59J7suts — Sen. Susan Collins (@SenatorCollins) January 27, 2020

At least four Republicans would have to vote alongside all Democratic senators to secure new testimony, which could lengthen the trial.

Romney, Collins, Lisa Murkowski of Alaska and Lamar Alexander of Tennessee are considered to be the most likely Republicans to vote for witnesses. Murkowski said Monday: "I've said before I'm curious about what Ambassador Bolton might have to say. I'm still curious."

Alexander said he won't decide until after both sides have answered questions from the Senate.

"After we've heard all the arguments, after we heard the questions and the answers to the questions and we've studied the record, then we'll have that vote. And at that time, I'll make a decision about whether I think they need additional evidence," he said.

Full coverage of President Donald Trump's impeachment trial

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Sen. Angus King, an independent from Maine who is aligned with the Democrats, said in an interview with NPR that he thinks more Republicans will want to hear from Bolton.

"I think there'll be more than four. My bold prediction will be five or 10," King said.

Sen. Lindsey Graham. R-S.C., a top Trump ally who's resisted calls for additional witnesses and documents, acknowledged that Bolton may be "a relevant witness" and said he'd consider subpoenaing a manuscript of his book.

"I want to know what's in the manuscript. Yeah, I think that's important," Graham told reporters.

Graham said that if witnesses are called, he also wants to include witnesses sought by the president.

"We're not going to get part of it. We're going to get all of it," he said.

Graham had said Sunday on Fox News that seeking witness testimony would "throw the country into chaos" and that the Senate should get testimony outside the impeachment process from witnesses Trump seeks, such as former Vice President Joe Biden, his son Hunter Biden and the whistleblower who alerted Congress to Trump's dealings with Ukraine.

Some Republicans have cast doubt on the bombshell report that Bolton alleges in his book that Trump directly linked withholding military aid to Ukraine with his push for investigations of Democrats.

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And they also said that if the Senate now votes to hear from witnesses like Bolton, senators had better allow for Trump's preferred witnesses, as well.

Trump, who has previously suggested that he would block Bolton's testimony, said last week at the World Economic Forum in Davos, Switzerland, that Bolton's appearing at the trial would present "a national security problem."

Sen. Josh Hawley, R-Mo., said Monday on "Fox & Friends" that if new testimony is approved, the Senate should hear from witnesses sought by Trump, too.

Hawley said that if additional testimony and documents are approved, he has prepared subpoenas for testimony and documents from the Bidens, the whistleblower and others.

Of Bolton's book, Hawley said, "It's certainly going to sell a lot" of copies.

"Listen, I can't tell from the New York Times report what is actually being reported here," he said. "I can't tell if this is something new. I can't tell if they've actually seen the manuscript. It's all a bunch of hearsay, and clearly it's an attempt to try to influence the course of the trial."

The New York Times reported that according to a manuscript of Bolton's book, which has not seen by NBC News, Trump told Bolton in August that nearly $400 million in aid to Ukraine would not be released until it provided all of the information it had in connection to the investigations of Democrats the president sought. One month earlier, Trump asked Ukrainian President Volodymyr Zelenskiy to investigate the Bidens and other Democrats.

Trump and his allies have said the investigations and aid were not linked, although acting White House chief of staff Mick Mulvaney said late last year that the aid was linked to an investigation of so-called Ukrainian electoral interference in 2016. Mulvaney later walked back those remarks.

Meanwhile, Trump's impeachment defense has argued in its "six facts" of the case that "not a single witness testified that the president himself said that there was any connection between any investigations and security assistance, a presidential meeting or anything else."

On Monday, Trump tweeted that he "NEVER" told Bolton of a link between investigations and aid, saying Bolton was making the assertion only "to sell a book."

Speaking on "Fox & Friends," Sen. John Cornyn, R-Texas, said the timing of the Bolton report "is a little interesting, isn't it?"

"But the best I can tell from what's reported in The New York Times [is] it's nothing different than what we've already heard, and as I said, no crimes were alleged, and these events never actually occurred, the withholding of aid and the investigations," Cornyn said.

Sen. Mike Braun, R-Ind., said Sunday on MSNBC's "Kasie DC" that, moving forward, it must be considered whether there was "anything that would motivate" Bolton to make the assertions.