Former national security adviser John Bolton John BoltonDiplomacy with China is good for America The Hill's Morning Report - Sponsored by The Air Line Pilots Association - Pence lauds Harris as 'experienced debater'; Trump, Biden diverge over debate prep DOJ launches probe into Bolton book for possible classified information disclosures MORE is adding fresh uncertainty to President Trump Donald John TrumpUS reimposes UN sanctions on Iran amid increasing tensions Jeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Trump supporters chant 'Fill that seat' at North Carolina rally MORE’s impeachment trial.

Bolton’s accusation, made in a forthcoming memoir, that Trump tied $391 million in aid to Ukraine to the country helping with investigations into Democrats — including former Vice President Joe Biden Joe BidenMomentum growing among Republicans for Supreme Court vote before Election Day Trump expects to nominate woman to replace Ginsburg next week Video of Lindsey Graham arguing against nominating a Supreme Court justice in an election year goes viral MORE and his son Hunter Biden — ricocheted across the Capitol.

Democrats quickly characterized the allegation as a “thunderbolt,” a “bombshell” and “gut check time” for Republicans, arguing that it underscored the need for additional witnesses and documents.

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Republicans, caught flat-footed, struggled to get on the same page, as some GOP lawmakers indicated Bolton’s allegation strengthened the need for witnesses, while others argued the memoir offered nothing that would change the outcome of the impeachment trial.

Asked if GOP senators had been in touch with the White House, Senate Republican Whip John Thune John Randolph ThuneWhat Senate Republicans have said about election-year Supreme Court vacancies The Hill's 12:30 Report - Presented by Facebook - Don't expect a government check anytime soon The Hill's Morning Report - Sponsored by The Air Line Pilots Association - Trump contradicts CDC director on vaccine, masks MORE (S.D.) told reporters that “my impression is that this sort of caught everybody by surprise.”

“Maybe you guys did because you're writing the stories, but, no, we did not know it was coming,” he added.

Sen. Lindsey Graham Lindsey Olin GrahamMomentum growing among Republicans for Supreme Court vote before Election Day Video of Lindsey Graham arguing against nominating a Supreme Court justice in an election year goes viral Warning signs flash for Lindsey Graham in South Carolina MORE (R-S.C.), facing a flurry of Bolton questions, acknowledged that he had no idea how portions of the manuscript were leaked.

“I have no idea how this happened. I don't know who leaked this. They say they didn't do it,” he added.

Bolton’s allegation is a significant curveball in Trump’s impeachment trial. GOP leadership and top aides had appeared increasingly confident they would be able to squash a vote, expected to take place on Friday, to open the door to calling new witnesses and compelling the administration to hand over documents.

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Democrats would need four Republicans to side with them in order to call a witness. They've requested four, including Bolton and acting White House chief of staff Mick Mulvaney Mick MulvaneyMick Mulvaney to start hedge fund Fauci says positive White House task force reports don't always match what he hears on the ground Bottom line MORE.

Sen. Mitt Romney Willard (Mitt) Mitt RomneyMomentum growing among Republicans for Supreme Court vote before Election Day Toobin: McConnell engaging in 'greatest act of hypocrisy in American political history' with Ginsburg replacement vote The Memo: Court battle explodes across tense election landscape MORE (R-Utah) set off alarm bells for Republicans when he told reporters that he thought it was “increasingly likely” that other members of his caucus would support hearing from Bolton. He threw in the caveat that he hadn’t spoken to his GOP colleagues.

“My thoughts about how we may all react are personal,” he said.

His statement earned him a public roasting by new Sen. Kelly Loeffler Kelly LoefflerTrump supporters chant 'Fill that seat' at North Carolina rally Vulnerable GOP incumbents embrace filling Supreme Court seat this year Georgia GOP Senate candidates cite abortion in pushing Ginsburg replacement MORE (R-Ga.), who previously donated to a 2012 pro-Romney super PAC. She accused the Utah senator of trying to “appease the left.”

But Romney was quickly backed up by Collins, who said in a statement that she thought Bolton’s allegations “strengthen the case” for witnesses. Collins reiterated that she will wait until after opening arguments and questions from both sides before she makes a decision but said she is “likely” to support calling witnesses.

Sens. Lamar Alexander Andrew (Lamar) Lamar AlexanderToobin: McConnell engaging in 'greatest act of hypocrisy in American political history' with Ginsburg replacement vote Chamber of Commerce endorses McSally for reelection Trump health officials grilled over reports of politics in COVID-19 response MORE (R-Tenn.) and Lisa Murkowski Lisa Ann MurkowskiMomentum growing among Republicans for Supreme Court vote before Election Day Collins: President elected Nov. 3 should fill Supreme Court vacancy Barrett seen as a front-runner for Trump Supreme Court pick MORE (R-Alaska), two other potential swing votes, were tight-lipped as they made their way through crowds of reporters.

“Well, I read it. I've said before I was curious what Ambassador Bolton might have to say, and I'm still curious,” Murkowski said Monday when asked by The Hill about her reaction to a New York Times story about Bolton’s forthcoming memoir.

She then paused to tell a reporter in front of her, “OK, you guys are going to have to move.”

A rotating cast of Republican senators was spotted going into McConnell's office ahead of the start of the trial on Monday as the caucus grappled with how to handle Bolton. In addition to normal members of leadership such as Thune and Sen. Roy Blunt Roy Dean BluntCDC tells Congress it urgently needs billion for vaccine distribution On The Money: Trump undercuts GOP, calls for bigger COVID-19 relief package | Communities of color hit hardest financially by COVID-19 | Businesses, states pass on Trump payroll tax deferral Trump undercuts GOP, calls for bigger COVID-19 relief package MORE (R-Mo.), Sens. Lindsey Graham (R-S.C.) and Mike Braun Michael BraunPessimism grows as hopes fade for coronavirus deal McConnell shores up GOP support for coronavirus package Patient Protection Pledge offers price transparency MORE (R-Ind.), part of a group that's taken over messaging for the caucus during the trial, were spotted heading into the leaders office.

Monday, the start of the second week of the trial, had been expected to be focused on Trump’s legal team, which is making its defense of Trump after a short two-hour preview on Saturday.

Instead, senators, House managers and Trump allies found themselves besieged with Bolton questions, underscoring how completely he had taken over the debate in the Capitol.

“What it's done is taken an already hot topic and added some fuel to the fire,” said Braun. “I'm not going to deny it's going to change the decibel level and probably the intensity at which we go about talking about witnesses.”

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Asked if the Times story had changed her thinking, Sen. Deb Fischer Debra (Deb) Strobel FischerHillicon Valley: Election officials prepare for new Russian interference battle | 'Markeyverse' of online fans helps take down a Kennedy | GOP senators unveil bill to update tech liability protections Google, Apple, eBay to meet virtually with lawmakers for tech group's annual fly-in Congress botched the CFPB's leadership — here's how to fix it MORE (R-Neb.) noted that the former administration official could release a statement if he had something to say because “it doesn't take a subpoena to put out a statement.”

When asked a similar question less than half an hour later, Fischer quipped, “Do you guys have memos on the same question to ask all the time? Just curious.”

A group of House Republicans advising Trump’s defense team got a similar reception. As they repeatedly sought to talk about the House managers and their impeachment case, the House GOP members were hit with question after question about Bolton.

“I would just say someone telling The New York Times what John Bolton’s draft manuscript supposedly says doesn’t change the key facts,” said Rep. Jim Jordan James (Jim) Daniel JordanSunday shows preview: Justice Ginsburg dies, sparking partisan battle over vacancy before election House passes resolution condemning anti-Asian discrimination relating to coronavirus Republicans call for Judiciary hearing into unrest in cities run by Democrats MORE (R-Ohio).

As multiple reporters yelled their questions at the group, Rep. Mike Johnson James (Mike) Michael JohnsonLWCF modernization: Restoring the promise Mike Johnson to run for vice chairman of House GOP conference Republicans call for Judiciary hearing into unrest in cities run by Democrats MORE (R-La.) responded, “Everyone just needs to take a deep breath.”

Democrats lapped up the renewed attention on the witness fight, using the Bolton frenzy to turn the spotlight back on Republicans.

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When a reporter tried to ask a nonwitness question during a Senate Democratic press conference, Senate Minority Leader Charles Schumer Chuck SchumerVideo of Lindsey Graham arguing against nominating a Supreme Court justice in an election year goes viral Graham signals support for confirming a Supreme Court nominee this year Pelosi orders Capitol flags at half-staff to honor Ginsburg MORE (D-N.Y.) joked, “Only for a moment.”

The frenzy is only likely to grow as the Senate heads toward a witness vote at the end of the week.

Sen. John Kennedy John Neely KennedyMORE (R-La.) walked away from reporters as they tried to quiz him on Bolton but not before dispensing some of his well-known folksy advice: Calm down.

“Given the fact ... that my Democratic friends have accused the president of just about everything except abandoning his children to wolves or hating little warm puppies, I’m not surprised that we’re having these last-minute leaks,” Kennedy said.

He added that “we’re about halfway through the trial. I think everybody ought to pop a Zoloft, take their meds.”