Congressional Democrats called for former national security adviser John Bolton to testify in Donald Trump’s impeachment trial following a new report that the president told Mr Bolton last August that he wanted to withhold military aid to Ukraine unless it aided investigations into the Bidens.

Mr Trump directly tied the withholding of nearly $400m (£305m) in military aid to the investigations of former vice president Joe Biden and his son Hunter in a conversation with Mr Bolton last summer, The New York Times reported on Sunday. That is according to an unpublished manuscript of Mr Bolton’s forthcoming book, the report said.

The book, The Room Where It Happened, is scheduled for publication on 17 March, but a White House review could attempt to delay its publication or block some of its contents.

Two people familiar with the book, who requested anonymity because of the sensitivity of the project, confirmed that it details Mr Trump tying aid to the desire for the Biden probes and details a number of conversations about Ukraine that he had with Mr Trump and key advisers, such as Mike Pompeo, the secretary of state. They said Mr Bolton is ready to testify in the Senate impeachment trial.

In a joint statement, the seven Democrat House impeachment managers called the report “explosive” and urged the Senate, controlled by Republicans, to agree to call Mr Bolton as a witness in Mr Trump’s trial, which kicks off its second full week on Monday. Mr Bolton has said he would testify before the Senate if subpoenaed.

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“The Senate trial must seek the full truth and Mr Bolton has vital information to provide,” the managers said in a statement following the report. “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 offences.”

Mr Trump is on trial, facing two charges – abuse of power and obstruction of Congress.

The assertion from Mr Bolton could undermine one core defence that has repeatedly been laid out by Mr Trump, his defenders and his legal team: that there was no explicit quid pro quo involved when the administration withheld the military assistance, as well as a White House visit coveted by Ukraine.

The White House has said that Mr Trump’s request for Ukrainian president Volodymyr Zelensky to investigate the Bidens, as well as a discredited theory that Ukraine interfered in the 2016 elections, was because he was interested in rooting out corruption and that he did nothing improper.

The president’s legal defence team is expected to mount a vigorous defence on Monday, when they deliver a full day of arguments against the impeachment charges.

The revelation from Mr Bolton’s book was certain to rile the dynamics of the trial this week, when the Senate was expected to face a critical vote on whether to allow witnesses at all.

Charles Cooper, an attorney for Mr Bolton, said he submitted the manuscript to the National Security Council’s records management division on 30 December for a standard review process to examine potentially classified information. Mr Cooper said they believed that the book manuscript did not include any classified material, and that its contents would not be shared with officials outside that review 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,” Mr Cooper said in the statement.

Sarah Tinsley, a spokeswoman for Mr Bolton, added: “The ambassador has not passed the draft manuscript to anyone else. Period.”

Senate majority leader Mitch McConnell and many Senate Republicans would prefer that the Senate avoid witnesses, but at least four GOP senators are seen as potential votes for favouring more testimony: Susan Collins of Maine, Lisa Murkowski of Alaska, Mitt Romney of Utah and Lamar Alexander of Tennessee.

Mr Romney and Ms Collins have indicated that they are likely to support hearing from witnesses and getting more evidence, and Mr Romney has said that he would like to hear from Mr Bolton.

“The odds of deposition for new witnesses is certainly rising dramatically,” one senior Republican official, who spoke on the condition of anonymity to candidly assess party dynamics, said after the publication of the New York Times report.

“John Bolton has the evidence,” Senate minority leader Chuck Schumer said in a tweet. “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.“

Earlier on Sunday, Mr Trump escalated his attacks on representative Adam Schiff, issuing what appears to be a veiled threat against the chairman of the House Intelligence Committee.

“Shifty Adam Schiff is a CORRUPT POLITICIAN, and probably a very sick man,” Mr Trump tweeted Sunday morning. “He has not paid the price, yet, for what he has done to our Country!”

Mr Schiff is the lead impeachment manager in the Senate trial.

Mr Schiff responded in an interview on NBC News’ Meet the Press, saying he believes that Mr Trump’s remarks were intended as a threat.

“This is a wrathful and vindictive president; I don’t think there’s any doubt about it,” Mr Schiff said in the interview. “And if you think there is, look at the president’s tweets about me today, saying that I should ‘pay a price.’ ”

“Do you take that as a threat?” host Chuck Todd asked.

“I think it’s intended to be,” Mr Schiff replied.

White House press secretary Stephanie Grisham said it was “ridiculous” for Mr Schiff to claim that Mr Trump was threatening him. In an appearance on Fox News Channel’s Media Buzz, she accused the California Democrat of “grandstanding,” although she acknowledged that she had not had an opportunity to ask Mr Trump what he meant by the tweet.

“I think he means . . . [Schiff] hasn’t yet paid the price with the voters,” Ms Grisham said.

She also echoed Mr Trump’s on Mr Schiff from earlier on Sunday, saying: “I mean, it seems he’s having a little bit of a mental issue when you sit on the floor for hours and hours and hours. He’s obsessed with this president and trying to take him down.”

Democrats contend that Mr Trump has continued to publicly solicit foreign interference in US elections and that the integrity of the 2020 race is at risk. The president fired back on Sunday by levelling the same accusation at his political opponents.

“The Impeachment Hoax is a massive election interference the likes of which has never been seen before,” he said in a tweet.

Some Republicans defended Mr Trump’s remarks about Mr Schiff. In an interview on CNN’s State of the Union, senator James Lankford said he was not troubled by Mr Trump’s declaration that Mr Schiff “has not paid the price.”

“I don’t think it’s a death threat. I don’t think he’s encouraging a death threat,” Mr Lankford said.

Host Jake Tapper responded by saying that “people who are supporters of the president have heard his rhetoric and then actually tried to bomb and kill politicians and the media.”

This prompted Mr Lankford to refer to the 2017 congressional baseball shooting that targeted Republicans and injured several people, including House minority whip Steve Scalise.

“So to be able to say the president’s trying to be able to spur this on would be able to say Democrats were trying to spur on the killing” of Republicans, Mr Lankford said.

Representative Zoe Lofgren, who is also an impeachment manager, called Mr Trump’s tweet about Mr Schiff “really unfortunate” and said the president has said things before “that seem threatening to people”.

“He really ought to get a grip and be a little more presidential,” she said on State of the Union.

In a tweet later on Sunday morning, Mr Trump also took aim at Mr Todd, accusing the Meet the Press host of holding a “softball interview” with Mr Schiff and “never even calling Shifty out on his fraudulent statement to Congress, where he made up ALL of the words of my conversation with the Ukrainian President!”

Both sides continue to spar over the question of whether the Senate trial will include witnesses. Some key Senate Republicans, already hesitant on the issue, became even more so over the weekend after Mr Schiff referred to a CBS News report in which an anonymous Trump ally was quoted as having warned politicians: “Vote against the president and your head will be on a pike.”