WASHINGTON — 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.

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.”

Trump is charged with abusing his power by asking Ukraine’s leader to help investigate Biden at the same the president was ordering that millions of dollars in aid be withheld — and then obstructing Congress in its probe.

They argued that Trump had legitimate reason to be concerned about Ukraine and, in any event, ultimately released the aid without the country committing to investigations the president wanted.

Democrats say Trump did so only after a whistleblower submitted a complaint about the episode.

The lawyers also defended the actions of Rudy Giuliani, the president’s personal lawyer whose efforts pressing for the Biden investigation helped lead to the firing of the American ambassador to Ukraine.

And they attacked the business dealings of Hunter Biden, who sat on the board of a Ukraine gas company at the same time his father was vice president, as they made the case that Trump was right to seek an investigation.

Ken Starr, whose independent counsel investigation into President Bill Clinton resulted in his impeachment before he was acquitted by the Senate, bemoaned what he said was an “age of impeachment.”

Impeachment, he said, requires both 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.”

Even as defense lawyers laid out their case as planned, it was clear that 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 him with investigations into Biden.

Trump’s legal team has repeatedly insisted otherwise.

The president also tweeted on Monday that he never told Bolton such a thing.

Republican senators face a pivotal moment, and 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 appeared unmoved by news of the Bolton book, telling Republicans they would take stock after the defense team concludes arguments.

McConnell’s 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 no later than Tuesday, senators have 16 hours for questions to both sides.

By late in the week, they are expected to hold a vote on whether 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, possibly later this week.