WASHINGTON (AP) — Senators faced mounting pressure Monday to summon John Bolton to testify at President Donald Trump’s impeachment trial even as Trump’s lawyers mostly brushed past extraordinary new allegations from the 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 Trump had wanted to withhold military aid from Ukraine until it helped with investigations into Democratic rival Joe Biden. That assertion could undercut a key defense argument — that Trump never tied the suspension of security aid to political investigations.

The revelation clouded White House hopes for a swift end to the impeachment trial, fueling Democratic demands for witnesses and possibly pushing more Republican lawmakers to agree. It also distracted from hours of arguments from Trump’s lawyers, who declared anew that no witness has testified to direct knowledge that Trump’s delivery of aid was contingent on investigations into Democrats. 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,” Trump attorney Jay Sekulow said. “We do not deal with speculation.”

Trump is charged with abusing his presidential power by asking Ukraine’s leader to help investigate Biden at the same time his administration was withholding hundreds of millions of dollars in security aid. A second charge accuses Trump of obstructing Congress in its probe.

Republicans are to conclude their arguments Tuesday.

On Monday, Trump’s attorneys, including high-profile lawyers Ken Starr and Alan Dershowitz, launched a historical, legal and political attack on the entire impeachment process. They said there was no basis to remove Trump from office, defended his actions as appropriate and assailed Biden, who is campaigning for the Democratic nomination to oppose Trump in November..

Former Florida Attorney Ggeneral Pam Bondi devoted her presentation to Biden and his son, Hunter, who served on the board of a Ukraine gas company at the same time his father was leading the Obama administration’s diplomatic dealings with Kyiv. The legal team argued that Trump had legitimate reasons to be suspicious of the younger Biden’s business dealings and concerned about corruption in Ukraine and that, in any event, he ultimately released the aid without Ukraine committing to investigations the president wanted.

Democrats say Trump released the money only after a whistleblower submitted a complaint about the situation.

Trump has sought, without providing evidence, to implicate the Bidens in the kind of corruption that has long plagued Ukraine. Though anti-corruption advocates have raised concerns, there has been no evidence of wrongdoing by either the former vice president or his son.

Starr, whose independent counsel investigation into President Bill Clinton resulted in his impeachment — he was acquitted by the Senate — bemoaned what he said was an “age of impeachment.” Impeachment, he said, requires an actual crime and a “genuine national consensus” that the president must go. Neither exists here, Starr said.

“It’s filled with acrimony and it divides the country like nothing else,” Starr said of impeachment. “Those of us who lived through the Clinton impeachment understand that in a deep and personal way.”

Dershowitz, the final speaker of the evening, argued that impeachable offenses require criminal-like conduct, though that view is largely rejected by legal scholars. He said “nothing in the Bolton revelations, even if true, would rise to the level of an abuse of power or an impeachable offense.”

“Purely non-criminal conduct, including abuse of power and obstruction of Congress, are outside the range of impeachable offenses,” Dershowitz said.

Elizabeth Warren, a presidential campaigner like Biden but also a Senate juror, told reporters she found Dershowitz’s arguments “nonsensical.”

Even as defense lawyers laid out their case as planned, it was clear Bolton’s book had scrambled the debate over whether to seek witnesses. Bolton writes that Trump told him he wanted to withhold security aid from Ukraine until it helped with investigations. Trump’s legal team has insisted otherwise, and Trump tweeted Monday that he never told Bolton such a thing.

“I NEVER told John Bolton that the aid to Ukraine was tied to investigations into Democrats, including the Bidens,” Trump said. “If John Bolton said this, it was only to sell a book.”

Republican senators face a pivotal moment. Pressure is mounting for at least four to buck GOP leaders and form a bipartisan majority to force the issue. Republicans hold a 53-47 majority.

“John Bolton’s relevance to our decision has become increasingly clear,” GOP Sen. Mitt Romney of Utah told reporters. Sen. Susan Collins of Maine said she has always wanted “the opportunity for witnesses” and the report about Bolton’s book “strengthens the case.”

At a private GOP lunch, Romney made the case for calling Bolton, according to a person unauthorized to discuss the meeting and granted anonymity.

Other Republicans, including Sen. Pat Toomey of Pennsylvania, said if Trump’s former national security adviser is called, they will demand reciprocity to hear from at least one of their witnesses. Some Republicans want to call the Bidens.

Senate Majority Leader Mitch McConnell didn’t know about Bolton’s book, his office said. But the GOP leader appeared unmoved by news of the Bolton book. His message at the lunch, said Indiana GOP Sen. Mike Braun, was, “Take a deep breath, and let’s take one step at a time.”

Once the president’s team wraps its arguments, senators have 16 hours for questions to both sides. By late in the week, they are expected to hold a vote on whether or not to hear from any witnesses.

While Democrats say Bolton’s revelations are reminiscent of the Watergate drip of new information, Republicans are counting on concerns subsiding by the time senators are asked to vote. They are being told that if there is agreement to summon Bolton, the White House will resist, claiming executive privilege.

That would launch a weeks-long court battle that could drag out the impeachment trial, a scenario some GOP senators would rather avoid.

Trump and his lawyers have argued repeatedly that Democrats are using impeachment to try to undo the results of the last presidential election and drive Trump from office.

Democrats, meanwhile, say Trump’s refusal to allow administration officials to testify only reinforces that the White House is hiding evidence. The White House has had Bolton’s manuscript for about a month, according to a letter from Bolton’s attorney.

Senate Democratic leader Chuck Schumer said: “We’re all staring a White House cover-up in the face.”

Rep. Adam Schiff, who leads the House prosecution team, called Bolton’s account a test for the senators.

“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,” Schiff said on CNN.

Bolton’s account was first reported by The New York Times and was confirmed to The Associated Press by a person familiar with the manuscript on the condition of anonymity. “The Room Where It Happened; A White House Memoir” is to be released March 17.

Joe Biden, campaigning in Iowa, said he sees no reason for testimony by him or his son.

“I have nothing to defend. This is all a game, even if they bring me up,” he told reporters. “What is there to defend? This is all — the reason he’s being impeached is because he tried to get a government to smear me and they wouldn’t. Come on.”

Trump said people could look at transcripts of his call with Ukrainian President Zelinskiy to see there was no pressure for investigations to get the aid. In that call, Trump asked Zelinskiy to “do us a favor” with the investigations as he was withholding nearly $400 million in military aid to the U.S. ally at war with Russia.

Democrats argued their side of the impeachment case last week, warning that Trump will persist in abusing his power and endangering American democracy unless Congress intervenes to remove him before the 2020 election.

Eventual acquittal is likely in a Senate where a two-thirds majority vote would be needed for conviction

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Associated Press writers Alan Fram, Mary Clare Jalonick, Andrew Taylor, Matthew Daly, Laurie Kellman and Padmananda Rama contributed to this report.