Bolton wrote in the forthcoming book that Trump told him that he wanted to withhold hundreds of millions of dollars in security aid from Ukraine until it helped him with politically charged investigations, including into Democratic rival Joe Biden.

Trump’s legal team has repeatedly insisted that the Republican president never tied the suspension of military assistance to the country to investigations that he wanted into Biden and his son.

The account immediately gave Democrats new fuel in their pursuit of sworn testimony from Bolton and other witnesses, a question expected to be taken up later this week by the Republican-led Senate. After Bolton situation, The impeachment trial resumed Monday afternoon with arguments from the defense team of Trump.

Bolton’s account was first reported by The New York Times. When the Times report went online Sunday night, the seven House Democratic managers immediately called on all senators to insist that Bolton be called as a witness and provide his notes and other relevant documents. Sen. Chuck Schumer, the Senate’s top Democrat, issued the same call.

Trump denied the claims in a series of tweets early Monday. “I NEVER told John Bolton that the aid to Ukraine was tied to investigations into Democrats, including the Bidens,” Trump said in a tweet. “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 people could look at transcripts of his call and statements by Ukraine President Volodymyr Zelinskiy that there was no pressure for such investigations to get the aid.

Bolton, who acrimoniously left the White House a day before Trump ultimately released the Ukraine aid on Sept. 11, has already told lawmakers that he is willing to testify, despite the president’s order barring aides from cooperating in the probe.

“Americans know that a fair trial must include both the documents and witnesses blocked by the President — that starts with Mr. Bolton,” the impeachment managers said in a statement.

While the Republican-controlled Senate is highly unlikely to remove Trump from office, it is important for him to try to blunt the Democratic accusations to limit political damage to his bid for a second term.

The impeachment trial rules provide for a two-step process on whether to subpoena witness and documents, with an initial vote on whether to consider doing so and, if approved, subsequent votes to actually call witnesses or demand documents.

Democrats argue this could allow Republicans to have it both ways – allowing them to first vote “yes” on whether to proceed and then vote “no” on actually allowing witnesses or documents.

As a result, vulnerable Senate Republicans could make the case to moderates that they had voted in favor of witnesses in the first vote while avoiding alienating Trump supporters by refusing to actually call any in later votes.

If the Senate called witnesses or demanded documents, the trial could lengthen. If not, the Senate could vote toward the end of the week on whether to remove Trump from office.

If that were to happen the trial could be over before the first US voting contest takes place in Iowa on Feb. 3 and before Trump is scheduled to deliver the annual State of the Union address to a joint session of Congress on Feb. 4.

AP and Reuters contributed to this story.