John Bolton facing fresh calls to testify in impeachment trial amid claims Trump wanted to withhold Ukraine aid

The US former national security advisor, John Bolton, is facing renewed pressure to testify in Donald Trump's impeachment trial after claims emerged the President told him he wanted to withhold security aid from Ukraine.

In a forthcoming book, due to be published in March, Bolton apparently wrote that Trump admitted he wanted to keep back hundreds of millions of dollars until the country 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 Ukraine 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.

The trial resumes this afternoon with arguments from Trump's defense 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 on the condition of anonymity to discuss the book, "The Room Where It Happened; A White House Memoir," ahead of its release March 17.

When the Times 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 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 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 September 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.

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

The lawyers revealed the broad outlines of their defense 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.

Trump's lawyers aren't expected to take as much time for their arguments as the Democrats, whose impeachment managers spoke for about 24 hours over three days.

But they also don't need to: Acquittal is likely in a Senate where Republicans hold a 53-47 majority, with a two-thirds vote needed for conviction.

Still, they see an opportunity to counter the allegations, defend the powers of the presidency and prevent Trump from being weakened politically ahead of November's election.

Trump faces two articles of impeachment. One accuses him of abusing his power by asking Ukraine to investigate Joe Biden, his Democratic rival, at the same time that his administration withheld hundreds of millions of dollars from the country.

The other alleges that Trump obstructed Congress by directing aides to not cooperate with the impeachment inquiry.