President Trump's former national security adviser John Bolton alleges in his forthcoming book that the president explicitly told him "he wanted to continue freezing $391 million in security assistance to Ukraine until officials there helped with investigations into Democrats including the Bidens," the New York Times first reported.

Why this matters: The revelations present a dramatic eleventh-hour turn in Trump's Senate impeachment trial. They directly contradict Trump's claim that he never tied the holdup of Ukrainian aid to his demands for investigations into his political opponent Joe Biden. Trump strongly denied Bolton's claims early Monday.

Senate Minority Leader Chuck Schumer has already seized on the story, tweeting: "John Bolton has the evidence. 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."

The seven House impeachment managers echoed those sentiments, saying in a joint statement: "There is no defensible reason to wait until his book is published, when the information he has to offer is critical to the most important decision senators must now make — whether to convict the president of impeachable offenses."

What's new: Sarah Tinsley, an adviser to Bolton, told me: "The ambassador's manuscript was transmitted to the White House for pre-publication review by the NSC. The ambassador has not passed the draft manuscript to anyone else. Period. It was sent over there, to the NSC, several weeks ago."

Tinsley would not comment on the Times' report of the contents of the book that's slated for release on March 17.

Bolton's attorney Charles Cooper said in a statement that he submitted a manuscript on behalf of his client on Dec. 30 to the National Security Council’s Records Management Division for standard prepublication security review for classified information, along with a cover letter.

"[W]e submitted the manuscript notwithstanding our firm belief that the manuscript contained no information that could reasonably be considered classified and on the assurance that the 'process of reviewing submitted materials is restricted to those career government officials and employees regularly charged with responsibility for such reviews' and that the 'contents of Ambassador Bolton’s manuscript will not be reviewed or otherwise disclosed to any persons not regularly involved in that process.'

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."

— Charles Cooper's statement to Axios

Behind the scenes: The New York Times learned about the contents of the Bolton manuscript before the White House press shop, per a source with direct knowledge.

Read Cooper's cover letter:

Editor's note: This article has been updated with the letter, Trump's denial, the book's release date and comments by Cooper and the House impeachment managers.