President Trump Donald John TrumpUS reimposes UN sanctions on Iran amid increasing tensions Jeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Trump supporters chant 'Fill that seat' at North Carolina rally MORE’s lawyers are poised to begin their second day of opening arguments in his Senate impeachment trial on Monday in what is expected to be a more robust presentation than their first day of abbreviated arguments over the weekend.

The sixth day of the trial will begin less than 24 hours after explosive new details about former national security adviser John Bolton John BoltonDiplomacy with China is good for America The Hill's Morning Report - Sponsored by The Air Line Pilots Association - Pence lauds Harris as 'experienced debater'; Trump, Biden diverge over debate prep DOJ launches probe into Bolton book for possible classified information disclosures MORE’s knowledge of the Ukraine affair threaten to severely complicate Trump’s defense.

The New York Times reported late Sunday that Bolton wrote in a draft copy of his forthcoming book that Trump told him in August that he wanted to suspend military assistance to Ukraine until the country helped with investigations into Democrats, including former Vice President Joe Biden Joe BidenMomentum growing among Republicans for Supreme Court vote before Election Day Trump expects to nominate woman to replace Ginsburg next week Video of Lindsey Graham arguing against nominating a Supreme Court justice in an election year goes viral MORE.

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Trump quickly denied telling Bolton the aid was tied to investigations into Democrats, alleging in a tweet that his former adviser was only making the allegations “to sell a book.”

It is unclear whether Trump's defense team will address the new details during Monday’s proceedings.

Bolton is one of a handful of witnesses Democrats have demanded should testify in the impeachment trial. The former national security adviser was called by the House as a witness but avoided testifying on instructions from the White House. He has since said he would be willing to testify before the Senate under subpoena.

Bolton’s reported account undercuts a key portion of the defense’s argument — namely that Trump did not tie the military assistance to investigations into 2016 election interference and the Bidens, as Democrats have argued. Even with Trump’s denial, the new information could make it more difficult for Republicans to resist calls for further witnesses.

“There can be no doubt now that Mr. Bolton directly contradicts the heart of the President’s defense and therefore must be called as a witness at the impeachment trial of President Trump,” the House Democratic impeachment managers said in a statement Sunday evening.

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“Senators should insist that Mr. Bolton be called as a witness, and provide his notes and other relevant documents. The Senate trial must seek the full truth and Mr. Bolton has vital information to provide,” they added.

But even if senators ultimately vote to call Bolton, a fight over executive privilege is expected. Trump signaled as recently as last week that he would fight Bolton’s testimony on executive privilege grounds because it could present problems for national security.

Trump claimed falsely Monday morning that House Democrats “never even asked” Bolton to testify and that it was not up to the Senate to call him as a witness now.

Senators return to Washington on Monday and will convene at 1 p.m. for the impeachment trial to consider articles approved by the Democrat-controlled House accusing Trump of abusing of power by pressuring Ukraine to launch investigations that could benefit his reelection campaign and obstructing the congressional investigation into his actions.

White House counsel Pat Cipollone and Trump’s personal attorney Jay Sekulow Jay Alan SekulowNow, we need the election monitors Judge denies Trump's request for a stay on subpoena for tax records Judge throws out Trump effort to block subpoena for tax returns MORE are leading figures in the president’s defense. However, Monday’s proceedings are expected to feature other prominent voices including Harvard Law professor emeritus Alan Dershowitz Alan Morton DershowitzDershowitz suing CNN for 0 million in defamation suit Bannon and Maxwell cases display DOJ press strategy chutzpah Ghislaine Maxwell attorneys ask for delay to unseal court documents due to 'critical new information' MORE and Ken Starr, the former independent counsel who investigated President Clinton.

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Dershowitz is a proponent of a contested argument that a crime is needed in order to impeach and remove a president from office, one that he is expected to put forth in arguing that House Democrats’ case fails to meet the constitutional standard and should be dismissed.

“Even if the factual allegations are true, which are highly disputed in which the defense team will show contrary evidence, but even if true, they did not allege impeachable offenses," Dershowitz, who has been an opinion contributor for The Hill, said on "Fox News Sunday." "So, there can't be a constitutionally authorized impeachment."

Dershowitz held the opposite position during the Clinton impeachment — something for which he has withstood scrutiny — but he has said that he has since changed his mind after doing more research.

Over roughly two hours on Saturday, Trump’s defense team sought to raise doubts about House Democrats’ case by painting it as incomplete and alleging they ignored evidence undercutting allegations that Trump abused his office by pressuring Ukraine to launch investigations into his political rivals. They also sought to attack the credibility of House Intelligence Committee Chairman Adam Schiff Adam Bennett SchiffTop Democrats call for DOJ watchdog to probe Barr over possible 2020 election influence Overnight Defense: Top admiral says 'no condition' where US should conduct nuclear test 'at this time' | Intelligence chief says Congress will get some in-person election security briefings Overnight Defense: House to vote on military justice bill spurred by Vanessa Guillén death | Biden courts veterans after Trump's military controversies MORE (D-Calif.), the lead impeachment manager and accused House Democrats of a partisan effort to remove Trump from the ballot in 2020.

The first day of the defense's opening arguments appeased Republicans, who are widely expected to vote to acquit the president, while Democrats claimed it strengthened their own demands for calling more witnesses to testify at the trial.

It’s unclear how long Trump’s lawyers plan to argue on Monday, though they’ve made clear they plan to be more brief than House impeachment managers, who used almost all of the 24 hours allotted to them over three days last week to lay out their case that Trump abused his power and obstructed Congress.

“I think we are going to be more efficient. I doubt there’s any scenario where we approach 24 hours presentation, but how that will be split over Monday and Tuesday is sort of hard to predict at this point,” a source working on the president’s legal team said Saturday.

It also remains uncertain to what degree the defense team plans to focus on potentially touchy topics such as former Vice President Joe Biden and his son Hunter, whom Trump asked Ukraine’s president to investigate during a July 25 phone call while repeating unfounded allegations about their dealings in the country.

Sekulow has indicated the lawyers plan to invoke the Bidens during their arguments, but they did not touch on the topic on Saturday. The president’s attorneys argued in a legal brief ahead of the trial that Trump had a legitimate reason to raise the Bidens and a debunked claim about Ukraine's involvement in 2016 election interference on the July call with Ukrainian President Volodymyr Zelensky.

Brett Samuels contributed to this report, which was updated at 10:48 a.m.