The lead impeachment manager for House Democrats said Monday the Senate cannot have a "meaningful" trial without receiving testimony from 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.

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.) told reporters shortly before the White House defense team was set to begin its second day of arguments that a New York Times report about Bolton's upcoming book, in which he reportedly claims 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 wanted to withhold Ukraine aid as leverage for investigations, underscores the need to call additional witnesses for testimony at the Senate trial.

"The news in the last 24 hours that John Bolton is not only prepared to testify, but that based on his manuscript that testimony would include a direct conversation with the president of the United States, where the president had made it clear he was conditioning military aid on political investigations or material that he wanted from Ukraine," Schiff said, "makes it all the more clear why you can't have a meaningful trial without witnesses and you certainly cannot have one without John Bolton."

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The Times reported Sunday that Bolton's book will say Trump wanted to continue freezing $391 million in U.S. aid to Kyiv until Ukrainian President Volodymyr Zelensky's administration agreed to launch two investigations that would benefit Trump politically, including a probe into a 2020 rival.

The report has sparked a wave of reactions on Capitol Hill, with Democrats stepping up their calls for witnesses to testify.

Some Republicans also appear to be increasingly open to the idea of hearing testimony from Bolton and other witnesses following news of the report.

Sen. Mitt Romney (R-Utah) said that “it’s very likely” he’ll vote for additional witnesses.

But it's unclear if Democrats will get the four GOP votes needed to call in such witnesses.

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Senate Majority Leader Mitch McConnell (R-Ky.) and other Republicans have been against having further testimony. Rather, they argue Democrats should not have sped through their impeachment inquiry and should've subpoenaed Bolton before the impeachment process moved to the trial phase.

Schiff has defended the decision to move forward, arguing Democrats would still be fighting the former Trump official in the courts for his testimony if they had gone down that path. Democrats argue that they had to move quickly because Trump was trying to "cheat" in the 2020 election by recruiting a foreign nation to damage the campaign of a political rival.

"This witness obviously has such relevant information to shed on the most egregious of all of the charges in the articles of impeachment and that is the president of the United States withheld military aid from an ally of war to help secure that nation's help to cheat in the next election," Schiff added.