National Security Advisor John R. Bolton listens as President Donald J. Trump meets with Prime Minister of the Netherlands Mark Rutte in the Oval Office at the White House on Thursday, July 18th, 2019 in Washington, DC.

Jabin Botsford | The Washington Post | Getty Images

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John Bolton to testify at President Donald Trump’s impeachment trial after an explosive report alleged that in his unpublished book, he said Trump personally tied Ukraine aid to an investigation of the Bidens — an account that conflicts with the president’s.

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

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.

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.

More from NBC News:

Republicans say they have no concerns about Trump-Parnas tape

Trump suggests Schiff will pay a ‘price’ for pushing impeachment

Trump’s Senate impeachment trial: What happened on Day Four

House Speaker Nancy Pelosi, D-Calif., tweeted that with the news Bolton reportedly had firsthand knowledge of Trump’s decision that ran counter to the White House account of the president’s actions, the “refusal of the Senate to call for him, other relevant witnesses, and documents is now even more indefensible.”

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.”

Sen. Amy Klobuchar, D-Minn., said while campaigning on Sunday in Des Moines, Iowa, “I don’t know how my Republican colleagues cannot call for witnesses…We should all be calling for witnesses. We have to get to the truth.”

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 Joe 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.

An aide to Bolton did not confirm or deny the substance of the report on Sunday night.

“Several weeks ago, the ambassador sent a hard copy of his draft manuscript to the White House for prepublication review by the National Security Council,” said Sarah Tinsley. “The ambassador has not passed that manuscript to anyone else for review. Period.”

The White House did not immediately respond to an NBC 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 press conference at the World Economic Forum in Davos, 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?”

Read More