WASHINGTON (AP) — Pressure increased Monday on senators to call John Bolton to testify at President Donald Trump's impeachment trial even as defense lawyers brushed past extraordinary new allegations from Trump's former national security adviser and made legal and historical arguments for acquittal.

The president's legal team resumed its presentation as 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 the country help investigate political rival Joe Biden.

The attorneys did not make any direct reference to revelations about Bolton's book draft as they opened arguments, though one did appear to take an oblique swipe.

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

Ken Starr, the former independent counsel whose investigation into President Bill Clinton resulted in his impeachment, 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.

Impeachment is "filled with acrimony and divides the country like nothing else," Starr said, adding, "Those of who lived through the Clinton impeachment understand that — and indeed in a personal way."

Even as defense lawyers laid out their case, it was clear that Bolton's book has scrambled the debate over whether to seek witnesses. In it, Bolton writes that Trump told him 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, and Trump tweeted on Monday that he never told Bolton such a thing.

Republican senators faced a pivotal moment as they arrived on Capitol Hill to resume Trump's trial. Democrats are demanding sworn testimony from Bolton and other key witnesses, and pressure is mounting on at least four Republicans to buck GOP leaders and form a bipartisan majority to force the issue.

"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."

The push for Bolton to testify did not appear to move Senator Majority Leader McConnell, who told Republicans at a closed lunch Monday they would take stock after the defense team concludes arguments.

"His message is what has been all along: Let's get through the next step," said Indiana GOP Sen. Mike Braun exiting the lunch. "That was it. 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 Thursday they are expected to hold a vote on whether or not to hear from any other witnesses. Republicans said if Bolton is called they will demand reciprocation, which could mean trying to call Biden and his son, who was on the board of a Ukrainian gas company.

Trump's team laid out the broad outlines of its defense in a rare Saturday session, at which 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.

Besides Starr, Monday's presentation was expected to include appearances by Alan Dershowitz, who will argue that impeachable offenses require criminal-like conduct, and former Florida Attorney General Pam Bondi.

Democrats, meanwhile, are saying that Trump's refusal to allow administration officials to testify in the impeachment proceeding 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."

Schumer drew on polls that show the public wants to hear from witnesses. "We want the truth," he said. "So do the American people."

Rep. Adam Schiff, who is leading the House prosecution team, called Bolton's account a test for the senators sitting as jurors.

"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.

Four Republicans would have to break ranks to join Democrats to call any witnesses, which would extend the trial, which has been expected to conclude fairly rapidly. The Republicans hold a 53-47 Senate majority.

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.

John Ullyot, a spokesman for the National Security Council that Bolton used to lead, said the manuscript was submitted to the NSC for "pre-publication review" and had been under initial review.

"No White House personnel outside NSC have reviewed the manuscript," he said.

Trump denied Bolton's claims in tweets early Monday.

"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."

Trump said people could look at transcripts of his call with Ukrainian President Vlodymyr Zelinskiy to see there was no pressure for such 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.

Trump falsely claimed Monday that the Democrat-controlled House "never even asked John Bolton to testify." Democrats did ask Bolton to testify, but he didn't show up for his deposition. They later declined to subpoena Bolton, as they had others, because he threatened to sue, which could lead to a prolonged court battle.

Schiff said Bolton — known to be a copious notetaker — should also provide documents.

Bolton, who sent this book manuscript to the White House for review, is now enmeshed in a legal dispute with the White House over the manuscript's use of direct quotes and other material from meetings and foreign leader discussions. That's according to a person familiar with the matter, who spoke on condition of anonymity because the person wasn't authorized to speak on the record.

The White House has requested that Bolton remove material it considers classified, the person said, which has the book behind schedule.

Bolton acrimoniously left the White House a day before Trump ultimately released the Ukraine aid on Sept. 11. He has already told lawmakers that he is willing to testify, despite the president's order barring aides from cooperating in the probe.

Eventual acquittal is likely in a Senate where a two-thirds majority vote would be needed for conviction. Still, the White House sees its Senate presentation this week as 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 while withholding military aid. The other alleges that Trump obstructed Congress by directing aides to not cooperate with the impeachment inquiry.

Democrats argued their side of the impeachment case for three days 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.