A DRAFT OF a book from former US national security adviser John Bolton appears to undercut a key defence argument in the impeachment trial of US President Donald Trump.

Bolton writes 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 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 evidence from Bolton and other witnesses, a question expected to be taken up later this week by the Republican-led Senate.

The trial resumes this afternoon with arguments from Trump’s defence team.

Bolton’s account was first reported by The New York Times and confirmed to The Associated Press by a person familiar with the manuscript of the book entitled The Room Where It Happened; A White House Memoir, ahead of its release on 17 March.

When the report went online last 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.

Senator Chuck Schumer, the Senate’s top Democrat, issued the same call. Trump denied the claims in a series of tweets.

“I NEVER told John Bolton that the aid to Ukraine was tied to investigations into Democrats, including the Bidens,” he tweeted.

“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 Vlodymyr 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 11 September, has already told members of Congress that he is willing to give evidence, despite the president’s order barring aides from co-operating 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.

First, though, Trump’s legal team will begin laying out its case in depth, turning to several high-profile lawyers to argue against impeachment.

The lawyers revealed the broad outlines of their defence in a rare but truncated Saturday session, where they accused House Democrats of using the impeachment case to try to undo the results of the last presidential election and drive Trump from office.

The legal team is expected to pick up on that theme and also dive into areas that received negligible attention during the Democrats’ presentation, including the now-concluded investigation into ties between Russia and Trump’s 2016 campaign.