Senators faced mounting pressure Monday to summon John Bolton to testify at President Donald Trump's impeachment trial even as Trump's lawyers brushed past extraordinary new allegations from Trump's former national security adviser and focused instead on corruption in Ukraine and historical arguments for acquittal.

Outside the Senate chamber, Republicans grappled with claims in a forthcoming book from Bolton that undercut a key defense argument — that Trump never tied withholding military aid to Ukraine to his demand that the country help investigate political rival Joe Biden.

The revelation clouded White House hopes for a swift end to the impeachment trial, fueling Democratic demands for witnesses and possibly pushing more moderate Republican lawmakers toward such testimony.

Watch the trial here.

It also distracted from hours of arguments from the Trump legal team, who declared anew that no witness has testified to direct knowledge that Trump's delivery of aid was contingent on investigations into Democrats though Bolton appeared poised to say exactly that if called on by the Senate to appear.

“We deal with transcript evidence, we deal with publicly available information," attorney Jay Sekulow said. “We do not deal with speculation, allegations that are not based on evidentiary standards at all."

After taking only two hours to begin its case Saturday, Trump’s defense team began their second day of arguing the nation’s 45th president should not be removed from office.

But day six of Trump’s impeachment trial began under a cloud for Trump’s defenders. Reports surfaced Sunday that former National Security Adviser John Bolton had submitted a book manuscript to the White House for review in late December which included passages that might contradict Trump’s assertions regarding the Ukraine investigation.

That led to GOP senators seemingly opening the door to approving Bolton’s testimony.

"I think it's increasingly likely that other Republicans will join those of us who think we should hear from John Bolton," Sen. Mitt Romney (R-UT) told reporters on Capitol Hill.

"The reports about John Bolton's book strengthen the case for witnesses and have prompted a number of conversations among my colleagues," said Sen. Susan Collins (R-ME).

After Democrats took all eight hours over three days to hammer their point that Trump is guilty of high crimes and misdemeanors, as their articles of impeachment allege, the president's defense lawyers only spoke for two hours Saturday on the first of their three days.

The Trump legal team’s arguments in the rare Saturday session were aimed at rebutting allegations the president abused his power when he asked Ukraine to investigate political rival Joe Biden and then obstructed Congress as it tried to investigate.

On Monday, House Democratic impeachment managers renewed their demands that senators allow testimony that was not heard during December’s impeachment hearings.

A draft of Bolton’s new book may undercut a key defense argument — that Trump never tied withholding of aid to Ukraine to a demand the country investigate Biden.

President Donald Trump told his national security adviser he wanted to maintain a freeze on military assistance to Ukraine until it launched political investigations into his Democratic rivals, according to a report in The New York Times on Sunday.

Bolton writes in the forthcoming book Trump told him that he wanted to withhold hundreds of millions of dollars in security aid from Ukraine until it helped him with investigations into Biden. Trump’s legal team has repeatedly insisted otherwise.

On Monday, a person familiar with the case told The Associated Press the White House has had Bolton’s manuscript for at least a month and has challenged his use of certain material it considers classified.

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 GOP-led Senate.

Here are the key figures in President Donald Trump's impeachment trial. House Speaker Nancy Pelosi. She led the impeachment effort. Chief Justice John Roberts. He will preside over the trial. The Senate's political leaders - Majority Leader Mitch McConnell and Minority Leader Chuck Schumer. The president's legal defense team - White House counsel Pat Cipollone, Trump personal lawyer Jay Sekulow, Kenneth W. Starr, Alan Dershowitz, along with Robert Ray and Jane Raskin. House Democratic impeachment managers

“I don’t know how you can explain that you wanted a search for the truth in this trial and say you don’t want to hear from a witness who had a direct conversation about the central allegation in the articles of impeachment,” U.S. Rep. Adam Schiff, the House impeachment lead prosecutor, said Monday on CNN.

Trump’s lawyers are mounting a wide-ranging, aggressive defense asserting an expansive view of presidential powers and portraying Trump as besieged by political opponents determined to ensure he won’t be reelected in November.

“They’re asking you to tear up all the ballots across this country on your own initiative, take that decision away from the American people,” White House Counsel Pat Cipollone told senators Saturday. “And we can’t allow that to happen.”

Impeachment Trial Day 6: What you need to know

The president is accused of abusing his office by asking Ukraine to investigate former Vice President Biden and his son, Hunter, while allegedly withholding aid from a U.S. ally at war with bordering Russia. The second article of impeachment accuses him of obstructing Congress by refusing to turn over documents or allow officials to testify in the House probe.

»Read the best lines from President Trump’s Senate impeachment trial

Prosecutors on Thursday argued Trump abused power for his own personal political benefit ahead of the 2020 election, even as the nation’s top FBI and national security officials were publicly warning off the theory that it was Ukraine, not Russia, that interfered in the 2016 election.

7 Things to Know About How a Senate Impeachment Trial Works. 1. Senators take an oath to "do impartial justice.". Though Senate majority leader Mitch McConnell has stated, "I'm not an impartial juror," the wording of the required Constitutional oath is clear on the necessary impartiality. 2. A majority is needed in order to call a witness. President Donald Trump has indicated he'd like the whistleblower to be called as a witness, but more moderate Republicans could prevent such an occurrence. 3.

On Friday, Democrats wrapped their third and final day of arguments that Trump should become the nation’s first president to be removed from office.

After the president’s defense team concludes its arguments Tuesday, senators will then have 16 hours to ask written questions and another four hours for deliberations. They will also then decide whether they want to call witnesses to testify.

The Senate is heading this week toward a pivotal vote on Democratic demands for testimony from top Trump aides, including Bolton and acting chief of staff Mick Mulvaney. It would take four Republican senators to join the Democratic minority to seek witnesses, and party lines appear to be strongly holding.

Impeachment was established in the U.S. Constitution as a way to accuse a president of a crime and then hold a trial to determine if guilty. The first step requires a U.S. House member to introduce an impeachment resolution. The House speaker directs the judiciary committee to hold a hearing to decide whether to put the full measure to a vote by the full chamber. A majority of the committee must approve the resolution. If approved, it moves to a full vote on the House floor. If a majority of the House vot

The U.S. Constitution requires a two-thirds majority in the Senate, or 67 senators, to convict in an impeachment trial.

Republicans hold 53 seats in the Senate, while Democrats hold 45. However, two Independents — including presidential candidate Bernie Sanders of Vermont — regularly caucus with Democrats, giving the nation’s blue party 47 votes.

If the Senate votes along party lines regarding impeachment — as did the House — 20 Republican senators would have to join Democrats in convicting Trump and removing him from office.

House Democrats are drafting impeachment articles against President Trump. Speaker Nancy Pelosi made the announcement Dec. 5, 2019. The announcement came after hours-long testimony on Dec. 4, 2019, from four legal scholars on whether the president has committed impeachable offenses. Democrats could schedule an impeachment vote before the end of 2019. If adopted, President Trump will face a trial in the U.S. Senate in a presidential election year. The Senate is controlled by the GOP.

The first article of impeachment passed by the House charges Trump with abuse of power.

Democrats allege Trump “solicited the interference of a foreign government, Ukraine, in the 2020 U.S. presidential election. He did so through a scheme or course of conduct that included soliciting the government of Ukraine to publicly announce investigations that would benefit his reelection, harm the election prospects of a political opponent, and influence the 2020 U.S. president election to his advantage.”

The “election prospects of a political opponent” refer to Biden, currently a front-runner in a narrowing field of Democratic White House hopefuls.

The president “also sought to pressure the government of Ukraine to take these steps by conditioning official U.S. government acts of significant value to Ukraine on its public announcement of investigations.”

Democrats argue the president “used the powers of his presidency in a manner that compromised the national security of the United States and undermined the integrity of the United States democratic process. He thus ignored and injured the interests of the nation.”

How is your senator likely to vote on impeachment?