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WASHINGTON — Democrats stepped up their calls Sunday night for former national security adviser John Bolton to testify at President Donald Trump's impeachment trial after an explosive report alleged that in his unpublished book, he said Trump personally tied aid for Ukraine to an investigation of the Bidens — an account that conflicts with the president's.

"John Bolton has the evidence," Senate Minority Leader Chuck Schumer, D-N.Y., tweeted.

According to the manuscript, as reported by The New York Times on Sunday night, Trump told Bolton that nearly $400 million in aid to Ukraine would not be released until it offered assistance with investigations of Democratic targets, including former Vice President Joe Biden and his son, Hunter.

NBC News has not seen a copy of the manuscript or verified the report, which cited multiple sources familiar with Bolton's account.

The contents of the manuscript were described as a rough account of how Bolton would testify should he be called as a witness in the Senate trial. The prospect of new witnesses has been viewed as unlikely given most Republicans' reluctance to accept additional testimony.

Trump addressed the report in a series of tweets Sunday night. "I NEVER told John Bolton that the aid to Ukraine was tied to investigations into Democrats, including the Bidens. 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 said.

Hill Democrats said Sunday that the new report highlighted the urgency of a Senate request for Bolton's testimony — a move that would require several GOP votes.

"It's up to four Senate Republicans to ensure that John Bolton, Mick Mulvaney, and the others with direct knowledge of President Trump's actions testify in the Senate trial," Schumer tweeted. Mulvaney is Trump's acting chief of staff.

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House Speaker Nancy Pelosi, D-Calif., tweeted that because of the report that Bolton had firsthand knowledge of Trump's decision that ran counter to the White House's account, the "refusal of the Senate to call for him, other relevant witnesses, and documents is now even more indefensible."

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The House Democrats' impeachment managers said in a statement that there could be "no doubt now that Mr. Bolton directly contradicts the heart of the President's defense and therefore must be called as a witness at the impeachment trial of President Trump. Senators should insist that Mr. Bolton be called as a witness, and provide his notes and other relevant documents."

In a statement on Monday, NSC spokesman John Ullyot said, "Ambassador Bolton's manuscript was submitted to the NSC for pre-publication review and has been under initial review by the NSC. No White House personnel outside NSC have reviewed the manuscript."

Sen. Amy Klobuchar, D-Minn. — one of the senators running for the Democratic presidential nomination whose campaigns have been affected by the need to serve as jurors in the impeachment trial — echoed the sentiment Sunday.

"I don't know how my Republican colleagues cannot call for witnesses," she said while campaigning in Iowa, adding: "We should all be calling for witnesses. We have to get to the truth."

Biden, also campaigning in Iowa, told NBC on Sunday night that he did not have "any idea of what's in the book."

"But if it in fact contradicts Trump, it's not a surprise," he said.

The president's allies have said the aid delay was unconnected to Trump's requests that Ukrainian officials announce investigations that stood to undercut his domestic political opponents, including Biden.

In the unpublished book, Bolton is reported to allege that other administration officials, including Mulvaney, Secretary of State Mike Pompeo and Attorney General William Barr, were made aware of Trump lawyer Rudy Giuliani's unusual involvement in a shadow foreign policy in Ukraine well before it became a central element of the whistleblower complaint at the heart of the impeachment inquiry.

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Charles Cooper, an attorney for Bolton, appeared to confirm the substance of the report Sunday, saying the manuscript was submitted to the National Security Council last month for a standard review for classified information.

It is "clear, regrettably, from The New York Times article published today that the prepublication review process has been corrupted and that information has been disclosed by persons other than those properly involved in reviewing the manuscript," he said.

The White House did not immediately respond to a request for comment.

Last week, Trump expressed misgivings over the prospect of Bolton's testimony.

"The problem with John is that it's a national security problem," he told reporters at an impromptu news conference at the World Economic Forum in Davos, Switzerland, adding that Bolton "knows some of my thoughts, what I think about leaders — what happens if he reveals what I think about a leader and it's not very positive?"

Lauren Egan, Alex Moe and Hallie Jackson reported from Washington. Maura Barrett reported from Des Moines, Iowa.