John Kelly, the retired Marine general who served as Trump's chief of staff for 18 months, said Monday that he believes the reported allegations made by Bolton in an upcoming book.

"If John Bolton says that in the book, I believe John Bolton,” Kelly said at a lecture series appearance in Sarasota, Fla..

Kelly added, "John’s an honest guy. He’s a man of integrity and great character, so we’ll see what happens,” according to the Sarasota Herald-Tribune.

The former White House chief of staff said "the majority of Americans would like to hear the whole story," adding, "I think if there are people that could contribute to this, either innocence or guilt ... I think they should be heard.”

Bolton reportedly claims in the upcoming book that Trump linked Ukrainian aid with the investigations he sought into the Bidens in an August conversation with his former national security adviser. Trump denies having done so.

Senate Republicans are expected to meet after the impeachment trial adjourns Tuesday afternoon to discuss whether to call witnesses, four GOP aides told NBC News.

The meeting is for “starting to check the conference on witnesses,” a GOP leadership aide said. Other topics likely will be discussed as well.

The Senate Republican Conference's conversations on witnesses are taking on a new sense of urgency with Trump's defense team's arguments set to conclude Tuesday and the question-and-answer phase expected to start Wednesday. 

Once the 16 hours of senators' questions of the House managers and Trump's lawyers are finished (likely taking two days), there will be up to four hours of debate on whether to consider subpoenas for witnesses.

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Lead House manager Adam Schiff ripped into Trump's defense team Tuesday morning, saying they are afraid to let John Bolton testify because "he would tell, in a captivating way that the public would watch, the most pernicious part of the president's scheme — the withholding of hundreds of millions of dollars of military aid to an ally at war in order to coerce these sham investigations.

"They don't want the country to hear it," he continued. "They just want to sweep  it under the rug."

Appearing on appeared on MSNBC’s "Morning Joe," Schiff singled out arguments about Rudy Giuliani, who was central to the Ukraine saga, and those presented by Ken Starr and Alan Dershowitz.

"It's hard to know where to begin with the arguments that Giuliani is just a bit player here, even though, of course, the president brought up Rudy Giuliani, I think, more than anyone else in that phone call with Zelenskiy. It was 'talk to Rudy,'" Schiff said. The argument presented by Trump lawyer Pam Bondi that Giuliani is "merely a distraction" wasn't "the least bit credible," he added.

On Starr and Dershowitz, Schiff said, "I was leaning over to my staff counsel and basically saying, 'This is the same Ken Starr that we're talking about, right? This is the same Ken Starr?' And then you've got the debate between 60-year-old Alan Dershowitz and 81-year-old Alan Dershowitz. You know, the weight of their own legal team doesn't believe their legal constitutional theory."

"But I think, at the end of the day, it all boils down to this: You know, they're reading between the lines of their defense," Schiff continued. "It's  basically, 'Yeah, he did it, we know he did it, the president knows he did it. We just don't want the American people to see any more evidence that he did it.' 

Sen. Mitt Romney, R-Utah, said Tuesday that the idea of Democratic House managers and Trump's defense team each choosing one or more witnesses "has merit."

When asked by NBC News' Peter Alexander whether he is confident at least three other Republican senators would support hearing from witnesses, Romney said, "I don’t think they’re all settled, as a group or as individuals, as to exactly how they’re going to vote. But I’d like to hear from John Bolton, and I think the idea that’s been expressed in the media about having each side be able to choose a witness, or maybe more than one witness on a prepared basis, has some merit."

Romney added, "I think if you’re going to have one side call witnesses, the other side ought to be able to do the same."

Asked what will happen if Democrats don't go along with the idea, he said, "If you don’t have 51 votes, nothing happens."

Senate Minority Leader Chuck Schumer, D-N.Y., predicted Tuesday that the longer President Donald Trump’s impeachment trial goes on, “the more likely that more new evidence will come out that further implicates the president.”

Schumer said there had been a “steady drip, drip, drip of information” with “one explosive article after another coming out” — most recently The New York Times’ reporting on “several stunning chapters from (former national security adviser John) Bolton’s book.”

The minority leader said the steady stream of additional facts pertinent to the charges against Trump was “reminiscent of Watergate.”

Schumer also dismissed calls by Republican senators for testimony from Joe Biden’s son Hunter Biden during the trial.

“What can Hunter Biden tell us about the president’s conduct with Ukraine?” Schumer asked, or about Trump’s “obstruction of Congress,” he added.

“Nothing obviously,” he said.

It wouldn't be surprising to see extended wrap-up arguments from White House counsel Pat Cipollone and lawyer Jay Sekulow on Tuesday. (Note that by now, we’ve seen every attorney from the president’s legal team present at least once: Pam Bondi, Cipollone, Alan Dershowitz, Eric Herschmann, Jane Raskin, Robert Ray, Sekulow and Ken Starr.) Remember, Sekulow has repeatedly pledged to be “efficient” in the team’s presentations.

Timing-wise: A Republican close to the legal team told NBC News, “Bolton news makes those who wanted to vote for witnesses a lot less resolved. It makes the task for the Trump defense team more important to keep this moving quickly and be done this week.” The source also said the Bolton reporting "has sucked a lot of energy out of the room. They are having to build that back with a compelling case.”

Plus: A person close to Bolton told NBC News that a single hard copy of his book was delivered last month to the White House for the National Security Council to review. What happened to the copy of the book is unknown to Bolton’s team, but it appears copies of it were made. Bolton’s team submitted the book “in good faith” and now feels that process was corrupted. Bolton doesn’t intend to speak publicly about the Ukraine issue until questions about his potential testimony are resolved. If he were to testify, he plans to do so as a fact witness.

Have on your radar: More from the Bolton manuscript, per NYT: that he told Barr last year about concerns the president was effectively granting personal favors to the leaders of Turkey and China.