Republican senators on Monday said that they expect a crucial vote on whether to allow witnesses in the impeachment trial to take place on Friday.

“We’re going to get to the specific question of witnesses on Friday,” Sen. 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.) told reporters.

Sen. 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.), the No. 2 Senate Republican, also said that the vote would “presumably” take place on Friday.

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“It probably happens Friday or Saturday at the latest,” Thune added.

The upcoming witness vote comes as The New York Times reported on Sunday night that 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’s forthcoming memoir will say that Trump tied aid to Ukraine to the country helping with investigations into Democrats, including former Vice President Biden and his son Hunter Biden.

The report has injected uncertainty into whether the Senate will call witnesses.

Democrats have requested four witnesses in the impeachment trial, including Bolton. GOP leadership had appeared confident as of last week that the effort would fall short, paving the way for Trump’s quick acquittal.

But 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) said it was “increasingly likely” more Republican senators would support calling Bolton. He clarified later that his comments were not based on conversations with colleagues.

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) added that she thought Bolton’s allegations “strengthen the case” for witnesses, but she is waiting to make a decision until after opening arguments and questions from senators.

Every GOP senator, including Romney and Collins, voted against an effort to subpoena Bolton as part of the rules resolution that passed last week.