Sen. Lindsey Graham Lindsey Olin GrahamMcConnell locks down key GOP votes in Supreme Court fight Will Republicans' rank hypocrisy hinder their rush to replace Ginsburg? Momentum growing among Republicans for Supreme Court vote before Election Day MORE (R-S.C.) predicted on Tuesday that if the Senate opens the door to calling new witnesses, Republicans will vote to subpoena President Trump Donald John TrumpBiden leads Trump by 36 points nationally among Latinos: poll Trump dismisses climate change role in fires, says Newsom needs to manage forest better Jimmy Kimmel hits Trump for rallies while hosting Emmy Awards MORE's top targets.

“I'll make a prediction: There will be 51 Republican votes to call Hunter Biden, Joe Biden Joe BidenBiden leads Trump by 36 points nationally among Latinos: poll GOP set to release controversial Biden report Can Donald Trump maintain new momentum until this November? MORE, the whistleblower and the DNC staffer at a minimum,” Graham told reporters.

His comments come as 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 claim in his forthcoming memoir that President Trump tied Ukraine aid to the country opening up investigations into former Vice President Joe Biden and his son Hunter Biden.

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GOP leadership had hoped to end the trial with no witnesses, but Bolton's accusation, reported by The New York Times, has thrown the ability of the caucus to quickly end the trial into question.

Conservatives are warning that if four Republican senators side with Democrats to call Bolton, then Trump's legal team should get as many witnesses. Since Democrats would not support calling the Bidens or the whistleblower, Republicans would have to provide the 51 votes from their 53 members.

The Senate is expected to vote Friday on whether to call witnesses. If 51 senators support calling witnesses, that would open the door to both sides making motions for specific individuals. The Senate would then vote on those specific individuals.

Democrats have said they want to subpoena four individuals, including Bolton and acting 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.

Updated: 1:23 p.m.