President Donald Trump vented rage on Twitter just hours before his Senate impeachment trial was set to resume Monday, denying his ex-national security advisor John Bolton's reported claim that military aid to Ukraine was frozen in order to secure investigation into Trump's political opponents.

Trump's pushback came amid growing pressure for the Republican-majority Senate to allow witnesses to testify in the trial.

The New York Times reported Sunday that Bolton, in his upcoming book "The Room Where It Happened," wrote that Trump personally tied a nearly $400 million aid package to Kyiv to an investigation of former Vice President Joe Biden and his son Hunter. CNBC has not seen a copy of Bolton's manuscript.

But Trump flatly denied the account from Bolton, who left the White House in September amid a public dispute with the president over whether he resigned or was fired.

"I NEVER told John Bolton that the aid to Ukraine was tied to investigations into Democrats, including the Bidens," Trump tweeted early Monday morning. "In fact, he never complained about this at the time of his very public termination."

"If John Bolton said this, it was only to sell a book," Trump claimed.

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At the White House later Monday, Trump told reporters that he has not seen a copy of Bolton's manuscript. But he maintained that Bolton's reported allegations are "false!"

Democrats have held Bolton up as a key witness who must be heard in the trial. And the handful of Republicans seen as being moderate enough to side with the Democrats on witnesses appeared to be taking Bolton's reported claims seriously.

Republican Sen. Mitt Romney of Utah told reporters Monday morning that "I think it's increasingly likely that other Republicans will join those of us who think we should hear from John Bolton."

Sen. Susan Collins, R-Maine, released a statement saying the reports on Bolton's book "strengthen the case for witnesses."

They "have prompted a number of conversations with my colleagues," Collins said.

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Republicans hold a 53-47 majority in the Senate. Democrats need a simple, 51-vote majority to have their requests for witnesses and documents approved. But Republicans have compiled their own list of witnesses, including Hunter Biden and the whistleblower whose complaint about Trump's July 25 call with Ukraine's leader sparked the impeachment proceedings.

Senate Minority Leader Chuck Schumer, D-N.Y., said Monday morning that the new reports erase even a "shred of logic left to not hear witnesses and review the documents."

It remains highly unlikely that two-thirds of the Senate will vote to remove Trump, as the Constitution requires.

Democratic House managers, led by House Intelligence Chairman Adam Schiff, D-Calif., laid out their case last week for Trump's conviction and removal from office on charges of abuse of power and obstruction of Congress, both related to his Ukraine dealings. Trump's legal team began their defense on Saturday and are set Monday to continue arguing that the president did nothing wrong.