Senate Majority Leader Mitch McConnell told his caucus on Tuesday that it hasn't secured the votes necessary to block Democrats from calling witnesses to testify in the impeachment trial, The Wall Street Journal reported.

McConnell compiled a list of "yes," "no," and "maybe" votes but didn't show the count to his fellow senators, The Journal said.

But Republican leadership is confident it can win over the necessary GOP votes by Friday, CNN reported. The party says that calling witnesses would extend the trial indefinitely.

Pressure on Republicans has intensified after The New York Times reported on Sunday that former national security adviser John Bolton wrote in his forthcoming book that the president told him last year he would withhold military aid to Ukraine until the country investigated his political rivals.

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Senate Majority Leader Mitch McConnell told his caucus on Tuesday that it hasn't secured the votes necessary to block Democrats from calling witnesses to testify in President Donald Trump's impeachment trial, The Wall Street Journal reported.

McConnell told his fellow Republicans during a meeting on Tuesday afternoon that he didn't have enough Republican votes to quash the Democrat-led effort, a vote for which is planned for Friday, The Journal and other outlets reported.

McConnell compiled a list of "yes," "no," and "maybe" votes but didn't show the count to his fellow senators, The Journal said.

But Republican leadership is confident it can win over the necessary GOP votes by Friday, CNN reported. The party says calling witnesses would extend the trial indefinitely.

Senate Democrats want at least four key witnesses, including former national security adviser John Bolton and acting White House chief of staff Mick Mulvaney, to testify. The vast majority of Republicans oppose calling witnesses, but some GOP senators have expressed willingness to break with their party on the issue.

Pressure on Republicans has intensified after The New York Times reported on Sunday that Bolton wrote in his forthcoming book that the president told him last year he would withhold military aid to Ukraine until the Ukrainian president acceded to his demands for investigations into his political rivals.

Bolton's allegations contradict Trump's repeated claim that he didn't leverage the military aid to pressure Ukraine to investigate former Vice President Joe Biden and other Democratic opponents.

Trump's former chief of staff John Kelly said on Monday that he believed Bolton's allegations over Trump's denials. Kelly was among Trump's closest advisers for 18 months after a stint as secretary of Homeland Security.

But it remains unclear whether Democrats can gather the four GOP votes necessary to reach the 51-vote threshold to call witnesses.