WASHINGTON — The White House and Senate Republican leaders struggled on Monday to salvage their plans to push toward a quick acquittal of President Trump this week in his impeachment trial, after a new account by his former national security adviser corroborated a central piece of the case against him.
The newly disclosed revelations by John R. Bolton, whose forthcoming book details how Mr. Trump conditioned military aid for Ukraine on the country’s willingness to furnish information on his political rivals, angered key Republicans and reinvigorated a bid to call witnesses, which would prolong the trial and pose new dangers for the president.
A handful of Republicans from across the ideological spectrum appeared to be moving closer to joining Democrats in a vote to subpoena Mr. Bolton, even as their leaders insisted that doing so would only delay his inevitable acquittal.
“I think it’s increasingly likely that other Republicans will join those of us who think we should hear from John Bolton,” Senator Mitt Romney, Republican of Utah, told reporters. He later told Republican colleagues at a closed-door lunch that calling witnesses would be a wise choice politically and substantively.
As they opened the second day of their defense, Mr. Trump’s lawyers ignored the revelations from Mr. Bolton, reported on Sunday by The New York Times, which bolstered the case made by the House Democratic prosecutors that the president had repeatedly tied the security assistance to investigations he wanted. The assertion is at the heart of their abuse of power charge against Mr. Trump, which accuses him of using his position to gain foreign help in his re-election campaign.
Instead, the White House team doubled down with a defense that was directly contradicted by the account in Mr. Bolton’s book, due out in March. Mr. Trump’s lawyers told senators that no evidence existed tying the president’s decision to withhold security aid from Ukraine to his insistence on the investigations, which they have claimed were requested out of a concern for corruption.
“Anyone who spoke with the president said that the president made clear that there was no linkage between security assistance and investigations,” said Michael Purpura, the deputy White House counsel.
On their second full day of oral arguments, Mr. Trump’s legal team sought to turn the Democrats’ accusations on their head. They defended and played down the role of Rudolph W. Giuliani, the president’s personal lawyer who was at the center of Mr. Trump’s Ukraine pressure campaign, calling him a “shiny object” Democrats were brandishing to distract from a weak case. The president’s lawyers sought to raise doubts about former Vice President Joseph R. Biden Jr., and his son Hunter Biden, suggesting they were corrupt in an effort to bolster their claim that the president had a legitimate reason to demand that they be investigated.
And they continued to argue that Mr. Trump’s actions were far from impeachable.
Alan Dershowitz, a celebrity law professor, argued that the Constitution holds that impeachment is for “criminal-like behavior,” telling senators that the country’s founders “would have explicitly rejected such vague terms as ‘abuse of power’ and ‘obstruction of Congress’ as among the enumerated and defined criteria for impeaching the president.”
The theory has been rejected by most constitutional scholars.
But behind closed doors, Republicans were singularly focused on the revelations from Mr. Bolton, which stoked turmoil in their ranks and opened new cracks in their so far near monolithic support for the White House strategy of denying witnesses and rushing toward a final verdict, almost certain to be an acquittal.
Senator Mitch McConnell, Republican of Kentucky, sought to calm his colleagues at the private lunch, telling them to “take a deep breath” and not to leap to conclusions about how to proceed.
But according to people familiar with Mr. McConnell’s thinking, he was angry at having been blindsided by the White House about Mr. Bolton’s manuscript, which aides there have had since late December. The leader put out a rare statement saying that he “did not have any advance notice” of Mr. Bolton’s account.
Inside the gathering near the Senate floor, just before the trial got underway, Senator Patrick J. Toomey, Republican of Pennsylvania, told colleagues that he might be willing to support calling witnesses as long as the roster would include someone friendlier to Mr. Trump’s case, like Hunter Biden, according to people familiar with the gathering who were not authorized to discuss it. The idea appeared to be gaining broader currency among Republicans.
“My expectation is that were there to be testimony from Mr. Bolton, there would be testimony for someone on the defense side as well,” Mr. Romney said.
Even Senator Lindsey Graham, Republican of South Carolina and usually a reliable ally of the president’s, said that Mr. Bolton “may be a relevant witness” and that he would like to see a copy of Mr. Bolton’s manuscript.
At the White House, Mr. Trump raged throughout the morning at Mr. Bolton, accusing him of lying. Hosting Israeli leaders, the president told reporters that he had not seen the manuscript of the former adviser’s book but disputed its claims as “false.”
In a series of early-morning tweets hours before the trial resumed, the president accused Mr. Bolton of telling stories “only to sell a book” and defended his actions toward Ukraine as perfectly appropriate.
“I NEVER told John Bolton that the aid to Ukraine was tied to investigations into Democrats, including the Bidens,” President Trump wrote just after midnight.
But Mr. Trump later complained to associates that the presentations from his defense team were boring.
On the Senate floor, Mr. Trump’s lawyers followed the president’s lead, never mentioning Mr. Bolton’s claims and at one point appearing to suggest that they were immaterial.
“We deal with transcript evidence, we deal with publicly available information,” said Jay Sekulow, one of the president’s lawyers. “We do not deal with speculation, allegations that are not based on evidentiary standards at all.”
As the session progressed, Mr. Trump’s lawyers began their promised assault on Mr. Biden and his son, asserting that Mr. Trump demanded investigations of them because there was significant evidence that they were corrupt.
They methodically sought to undermine the case that House managers delivered over more than 22 hours last week. They argued that Mr. Trump said nothing wrong on a July 25 call with the president of Ukraine, never sought to leverage an Oval Office meeting, and did more to support Ukraine against Russian aggression than previous presidents.
“The managers have not met their burden, and these articles of impeachment must be rejected,” Eric Herschmann, one of the president’s lawyers, told senators.
In a somewhat improbable echo of the last presidential impeachment trial, Ken Starr, who relentlessly pursued President Bill Clinton for lying about an extramarital affair with a young aide, also appeared before the Senate to defend Mr. Trump. He argued that the president committed no impeachable offense and urged senators to “restore our constitutional and historical traditions,” in which impeachment was rare.
“Like war, impeachment is hell,” Mr. Starr told senators, casting himself as a skeptic of the constitutional remedy he enthusiastically pursued 21 years ago. “Or at least, presidential impeachment is hell.”
While it is not clear that Republicans will vote to call additional witnesses when they vote on the issue this week, the revelations from Mr. Bolton appeared to shift the dynamic that had taken hold at the end of last week’s arguments, when it appeared unlikely that Democrats would win the support of the four Republicans they need to force the issue.
On Monday, Democrats said they were newly optimistic that the momentum of the trial was pushing toward a vote for witnesses and documents, and they worked to increase the pressure on hesitant Republicans to embrace the moves.
“It boils down to one thing: we have a witness with firsthand evidence of the president’s actions for which he is on trial,” said Senator Chuck Schumer of New York, the Democratic leader. “He is ready and willing to testify. How can Senate Republicans not vote to call that witness and request his documents?”
Senator Susan Collins, Republican of Maine, who had previously indicated she would most likely support additional witnesses, said the revelations about Mr. Bolton’s book “strengthen the case for witnesses and have prompted a number of conversations among my colleagues.” Senator Lisa Murkowski, Republican of Alaska, said she was “curious” about what Mr. Bolton would say, but gave no hint of how she would vote on the matter.
But Republican leaders labored to play down the significance of Mr. Bolton’s account.
“The best I can tell from what’s reported in The New York Times, it is nothing different from what we have already heard,” Senator John Cornyn, Republican of Texas, said on Fox News.
Mr. Herschmann and Pam Bondi, another of Mr. Trump’s lawyers, delved deeply into Hunter Biden’s work on the board of Burisma, a Ukrainian energy company, at the time his father was vice president, suggesting it was improper for him to hold the post while his father served. Ms. Bondi also noted that the elder Mr. Biden had called for the removal of prosecutor who was looking into Burisma.
“What we are saying is that there was a basis to talk about this, to raise this issue,” Ms. Bondi said.
But it was United States policy at the time that the prosecutor, who was widely regarded as corrupt, should be removed. In a statement on Monday, Andrew Bates, the Biden campaign’s rapid-response director, said: “Here on Planet Earth, the conspiracy theory that Bondi repeated has been conclusively refuted.”
Later, Jane Raskin, one of the president’s lawyers, called Mr. Giuliani a “colorful distraction” in the case, arguing that the House impeachment investigators did not subpoena him to testify because they did not think he would back up their claims that he was executing a shadow foreign policy.
“In this trial, in this moment, Mr. Giuliani is just a minor player — that shiny object designed to distract you,” Ms. Raskin said.
Mr. Giuliani defied a House subpoena for documents. Legal experts suggest he would have refused to disclose any of his conversations with Mr. Trump on the basis of attorney-client privilege even if called to testify. And he would surely have been a difficult witness, given his often erratic performance in televised interviews.
Reporting was contributed by Catie Edmondson, Maggie Haberman, Sheryl Gay Stolberg and Patricia Mazzei.