Republicans in the Senate are facing new pressure to subpoena key witnesses on the impeachment trial of 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.

The Senate was headed into the second week of the trial facing a pivotal vote on the subject, and it looked like Democrats would almost certainly not win the four GOP votes needed to subpoena new witnesses.

But that was before a report Sunday night in The New York Times.

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The report, based on an unpublished manuscript by Trump's former national security adviser John Bolton John BoltonDiplomacy with China is good for America The Hill's Morning Report - Sponsored by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, said Bolton in his forthcoming book claims the president tied $391 million in aid to Ukraine to his requests for that country to investigate 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.

Democrats immediately pounced on the news, with the Democratic impeachment managers saying there was no excuse for GOP senators not to vote for witnesses.

Bolton is one of the witnesses most important to hear from, Democrats were saying even before the new report.

"Senators should insist that Mr. Bolton be called as a witness, and provide his notes and other relevant documents. The Senate trial must seek the full truth and Mr. Bolton has vital information to provide," the House managers said in a statement.

The White House was aware of the claims in Bolton's book, thought it is not clear for how long.

“Ambassador Bolton’s manuscript was submitted to the NSC for pre-publication review and has been under initial review by the NSC. No White House personnel outside NSC have reviewed the manuscript,” National Security Council spokesman John Ullyot said in a statement.

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Senate Republican leaders before the Bolton revelations had voiced confidence that they will keep their conference unified enough to defeat a motion to subpoena new evidence, which could allow the trial to wrap up at the end of the week.

The GOP is almost certain to lose the vote of Sen. Susan Collins Susan Margaret CollinsJeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Trump supporters chant 'Fill that seat' at North Carolina rally Momentum growing among Republicans for Supreme Court vote before Election Day MORE (R-Maine), and 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) on Saturday said that “it’s very likely” he’ll vote for additional witnesses.

The third and fourth GOP votes required by Democrats to win a majority have been seen as trickier gets.

Sen. 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) said Saturday she is reviewing her notes and dismissed speculation that she is leaning against new witnesses.

“There are a lot of people trying to divine tea leaves,” Murkowski quipped about the intense scrutiny over her statements.

Murkowski insisted she is keeping an open mind on voting for subpoenas for 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.

House Intelligence Committee Chairman Adam Schiff Adam Bennett SchiffTop Democrats call for DOJ watchdog to probe Barr over possible 2020 election influence Overnight Defense: Top admiral says 'no condition' where US should conduct nuclear test 'at this time' | Intelligence chief says Congress will get some in-person election security briefings Overnight Defense: House to vote on military justice bill spurred by Vanessa Guillén death | Biden courts veterans after Trump's military controversies MORE (D-Calif.), the lead manager of the House impeachment team, irritated Collins and other GOP senators when he said in his closing statement on Friday night that they feared crossing Trump.

Murkowski, however, said Schiff’s remarks wouldn’t factor in her decision making.

“I’ve taken a lot of notes — it takes me back to law school. What I haven’t done is I haven’t gone through any of those, but along the way I made little asterisks and notations about what I want to see, what questions I still have. So I have lot of work to do on my own,” she said.

Democrats had been growing more pessimistic about winning the witness vote, but the report in the Times gave new momentum to their calls.

Nancy Pelosi House Speaker(D-Calif.) said the Times report gave Republican senators a choice between "our Constitution or a cover-up."

"Amb. Bolton reportedly heard directly from Trump that aid for Ukraine was tied to political investigations. The refusal of the Senate to call for him, other relevant witnesses, and documents is now even more indefensible," Pelosi tweeted.

A vote could take place soon.

Trump’s defense team, which used only a couple hours of its allotted floor time on Saturday, will renew its arguments at 1 p.m. Monday but is not expected to use its full 24 hours.

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Senate Democrats say they plan to use the full 16 hours to ask questions after opening arguments, which sets up a debate Wednesday or Thursday on whether it should be in order to call for additional evidence.

If that motion fails, the trial could be wrapped up by the end of the week.

GOP leaders have warned their colleagues that Trump will invoke executive privilege over his conversations with Bolton and Mulvaney and that a court fight to settle it might drag the trial out for weeks.

Rep. Zoe Lofgren Zoe Ellen LofgrenTop Democrats call for DOJ watchdog to probe Barr over possible 2020 election influence DHS opens probe into allegations at Georgia ICE facility House passes legislation to boost election security research MORE (D-Calif.), one of the impeachment managers, admitted Sunday, before the Times report, that she has no idea what to expect from potential GOP swing votes such as Sen. 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.) or Sens. Cory Gardner Cory Scott GardnerJeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Momentum growing among Republicans for Supreme Court vote before Election Day Gardner on court vacancy: Country needs to mourn Ginsburg 'before the politics begin' MORE (R-Colo.) and Martha McSally Martha Elizabeth McSallyTrump supporters chant 'Fill that seat' at North Carolina rally Momentum growing among Republicans for Supreme Court vote before Election Day Vulnerable GOP incumbents embrace filling Supreme Court seat this year MORE (R-Ariz.), who both face reelection this year and potential primary challenges.

“As I speak and I sit there, I find myself looking at the senators — a lot of them I served with when they were in the House — and wondering what’s going through their minds,” she said on CNN’s “State of the Union” Sunday

Democrats say that vulnerable GOP incumbents will pay a political price in November’s general election if they vote against witnesses, pointing to recent polls showing strong public support for calling additional evidence, even among Republicans.

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But GOP incumbents also have to weigh the backlash from the GOP base if they vote to extend the trial of a president who has maintained strong approval ratings among Republican voters.

White House counsel Pat Cipollone argued Saturday that voting to remove Trump from office would be a far greater subversion of democracy than anything Democrats have charged Trump with.

“They’re asking you to remove President Trump from the ballot in an election that’s occurring in approximately nine months,” he said. “They’re here to perpetrate the most massive interference in an election in American history.”

Jordain Carney contributed.