Two Republican senators said Monday that new revelations from John Bolton bolstered their case for summoning witnesses in President Trump’s impeachment trial, as fresh allegations from the former White House official left Senate Republicans scrambling as they faced renewed pressure to seek new evidence.

In his forthcoming book, former national security adviser Bolton alleges that Trump directly tied the holdup of nearly $400 million in military aid to Ukraine to desired investigations of former vice president Joe Biden and his son Hunter.

The disclosure upended the Senate as the second full week of Trump’s impeachment trial began. Democrats insisted that the Senate must call the former White House official as a witness.

Trump faces two charges — abuse of power and obstruction of Congress.

Many senior Republicans dismissed details from Bolton’s book as nothing new — and nothing that would change Trump’s expected acquittal by the GOP-controlled Senate. A close Trump ally, Sen. Lindsey O. Graham (R-S.C.), said he wanted to see the unpublished manuscript of Bolton’s book or at least be briefed on it.

But two key GOP senators — Susan Collins (Maine) and Mitt Romney (Utah) — said they expect other Republicans to back a push for new evidence as part of the trial.

“I think it’s increasingly likely that other Republicans will join those of us who think we should hear from John Bolton,” Romney said Monday, repeatedly calling Bolton’s testimony “relevant.” “It’s important to be able to hear from John Bolton for us to be able to make an impartial judgment.”

Collins said reports about Bolton’s unpublished manuscript “strengthen the case for witnesses and have prompted a number of conversations among my colleagues.”

The book, “The Room Where It Happened,” is scheduled for publication March 17, but the White House could attempt to delay its publication or block some of its contents. The manuscript has been submitted to the National Security Council for a standard review of classified material, and an NSC spokesman said Monday that no White House official outside the NSC has reviewed it.

Two people familiar with the book, who spoke on the condition of anonymity because of the sensitivity of the project, confirmed that it details Trump tying aid to the desire for Biden probes and details a number of conversations about Ukraine that Bolton had with Trump and key advisers, such as Secretary of State Mike Pompeo.

But privately, some senior Republicans were left uncertain about how the revelations would ultimately affect a key vote expected later this week on whether to admit additional witnesses and testimony into the impeachment proceedings. GOP senators, apparently blindsided, have called the White House, asking who knew what and when, according to a senior administration official.

“This sort of caught everybody by surprise,” said Senate Majority Whip John Thune (R-S.D.).

Two other Republican senators being closely watched by Senate GOP leaders and the White House — Lamar Alexander (Tenn.) and Lisa Murkowski (Alaska) — were as noncommittal Monday on additional witnesses as they were before the news emerged about Bolton’s book, which was first reported by the New York Times on Sunday.

“I stated before that I was curious as to what John Bolton might have to say,” Murkowski said. “From the outset, I’ve worked to ensure this trial would be fair and that members would have the opportunity to weigh in after its initial phase to determine if we need more information.

She added: “I’ve also said there is an appropriate time for us to evaluate whether we need additional information — that time is almost here. I look forward to the White House wrapping up presentation of its case.”

Senate Majority Leader Mitch McConnell (R-Ky.) and most of his ranks have been aiming to wrap up Trump’s impeachment trial later this week, with the White House legal team finishing its arguments later Monday or Tuesday and then senators engaging in up to 16 hours of questions for both sides.

A protracted fight over witnesses and documents — should that key vote allow them later this week — could significantly drag out the length of Trump’s trial. One idea floated by Graham was for senators to look at Bolton’s unpublished manuscript. But for that to happen, Graham, said, the Bidens would also have to be investigated.

That was the consensus, Republicans said, after a closed-door party lunch, when senators concurred that if Bolton were called, the Senate would similarly have to summon the Bidens, according to Sen. Mike Braun (R-Ind.).

McConnell’s message to the conference was that, “remember, we passed a rules package that gives us an opportunity to vote on this very issue of witnesses after we hear both sides and ask our questions,” said Sen. Kevin Cramer (R-N.D.).

“That seemed wise at the time and seems even wiser now,” Cramer said.

Numerous other Republican senators — including Joni Ernst (Iowa), Rob Portman (Ohio), Mike Crapo (Idaho) and Shelley Moore Capito (W.Va.) — reserved judgment on Monday, particularly on whether the Bolton revelations make them more likely to favor calling him and other witnesses.

Thune said he did not “personally see it as a game-changer.”

“I think a lot of our members are going to look at it and say it doesn’t change the basic facts of the case, and I don’t think that it does,” he said.

Sen. John Barrasso (R-Wyo.) dismissed it as “nothing new.”

Meanwhile, some Republicans also began questioning the credibility and motives of Bolton — a longtime figure in GOP national security circles.

“You’ve got to keep in mind, for the first time in his life, he was fired,” said Sen. James M. Inhofe (R-Okla.). “That does have an effect on people.”

Bolton has said he resigned.

Sen. John Neely Kennedy (R-La.) lumped Bolton in with Lev Parnas, the indicted associate of Trump attorney Rudolph W. Giuliani who has turned over a trove of documents to congressional investigators and has implicated the president in the Ukraine dealings.

“Are Mr. Parnas and Mr. Bolton telling the truth? I don't know,” Kennedy said. “But given the circumstances under which they are making the allegations, I wouldn't bet my house on it. And if I were betting your house, it would only be a maybe.”

Even as they used the latest revelations to pressure Republicans on calling additional witnesses, some senior Democrats were skeptical that enough GOP senators would ultimately acquiesce.

“I think he has probably reached the point where he has fewer than four who will vote for the motion for witnesses and believes that he ultimately has all the votes he needs to exonerate this president,” Sen. Richard J. Durbin (D-Ill.), the second-ranking Senate Democrat, said of McConnell. “If that’s what these members of the Senate want to carry home from this experience, I think they’re going to be surprised at the reaction.”

Erica Werner, Karoun Demirjian and Paul Kane contributed to this report.