WATCH: Explosive claims in John Bolton's manuscript cloud trial
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.
In the trial itself, Trump's attorneys 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 summoned by the Senate.
"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."
WATCH: Bombshell development involves John Bolton, one of Trump's closest men
The legal team took turns decrying the impeachment, which accuses Trump of abusing his power by asking Ukraine's leader to help investigate Biden at the same time he was ordering that millions of dollars in aid be withheld - and then of obstructing Congress in its probe.
Michael Purpura, a deputy White House counsel, argued that Trump had legitimate reason to be concerned about corruption in 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.
Ken Starr, whose independent counsel investigation into President Bill Clinton resulted in his impeachment - Clinton 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.
Impeachment "is filled with acrimony and it divides the country like nothing else," Starr said. "Those of us who lived through the Clinton impeachment understand that in a deep and personal way."
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.
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, and Trump 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."
Senator Majority Leader McConnell appeared unmoved, telling 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 late in the week, 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.
Democrats, meanwhile, say 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."
Rep. Adam Schiff, who leads 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.
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.
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 Volodymyr 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.
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.
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.