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

The New York Times reported Sunday evening that in last summer’s conversation, Trump directly tied the holdup of nearly $400 million in military assistance to the investigations of former vice president Joe Biden and his son Hunter Biden. That is according to an unpublished manuscript of Bolton’s forthcoming book, the Times said.

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

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

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

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

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

The assertion from Bolton could undermine one core defense that has repeatedly been laid out by 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 president responded to the report early Monday, tweeting “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.”

The White House has said that 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 defense team is expected to mount a vigorous defense on Monday when they deliver a full day of arguments against the impeachment charges.

The revelation from the Bolton book was certain to roil 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, a lawyer for Bolton, said he submitted the manuscript to the National Security Council’s records management division on Dec. 30 for a standard review process to examine potentially classified information. 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,” Cooper said in the statement.

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

Senate Majority Leader Mitch McConnell (R-Ky.) and many Senate Republicans would prefer the Senate avoid witnesses, but at least four GOP senators are seen as potential votes for favoring more testimony: Susan Collins (Maine), Lisa Murkowski (Alaska), Mitt Romney (Utah) and Lamar Alexander (Tenn.).

Romney and Collins have already indicated that they are likely to support hearing from witnesses and getting more evidence, and Romney has also said that he would like to hear from 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 Sunday evening after the publication of the Times report.

“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,” Senate Minority Leader Charles E. Schumer (D-N.Y.) said in a tweet.

Earlier Sunday, Trump escalated his attacks on Rep. Adam B. Schiff (D-Calif.), 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,” Trump tweeted Sunday morning. “He has not paid the price, yet, for what he has done to our Country!”

Schiff is the lead impeachment manager in the Senate trial.

Schiff responded in an interview on NBC News’s “Meet the Press,” saying he believes that 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,” 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,” Schiff replied.

White House press secretary Stephanie Grisham said it was “ridiculous” for Schiff to claim that 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 Trump what he meant by the tweet.

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

She also echoed Trump’s attack earlier Sunday on Schiff, 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 Trump has continued to publicly solicit foreign interference in U.S. elections and that the integrity of the 2020 race is at risk. The president fired back Sunday by leveling 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 on Sunday defended Trump’s remarks about Schiff. In an interview on CNN’s “State of the Union,” Sen. James Lankford (R-Okla.) said he was not troubled by Trump’s declaration that 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,” 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 Lankford to refer to the 2017 congressional baseball shooting that targeted Republicans and injured several people, including House Minority Whip Steve Scalise (R-La.).

“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, Lankford said.

Rep. Zoe Lofgren (D-Calif.), who is also an impeachment manager, called Trump’s tweet about 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 Sunday morning, Trump also took aim at Todd, accusing the “Meet the Press” host of holding a “softball interview” with 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 Schiff referred to a CBS News report in which an anonymous Trump ally was quoted as having warned lawmakers, “Vote against the president and your head will be on a pike.”

Tom Hamburger contributed to this report.