Republicans turn on Bolton as his account scrambles the impeachment trial.

For years, John R. Bolton has been a conservative Republican stalwart: foreign policy hawk, ambassador to the United Nations under President George W. Bush, fellow at the American Enterprise Institute, commentator on Fox News and national security adviser to President Trump.

On Monday, Mr. Bolton, who left Mr. Trump’s White House last summer, appeared on the verge of becoming a Republican pariah. Hours after The New York Times published details from his forthcoming book, in which he wrote that Mr. Trump told him he would not unfreeze military aid to Ukraine until that country investigated his political rivals, Mr. Bolton’s fellow Republicans began turning on him.

“Well, there’s a so-called blockbuster report in today’s New York Times — it’s a story about selective leaks from a book that you can pre-order on from John Bolton,” said Senator John Barrasso, Republican of Wyoming, appearing to suggest that Mr. Bolton is merely trying to sell books.

“The timing is a little interesting, isn’t it?” Senator John Cornyn, Republican of Texas, commented on Fox News.

Mr. Trump himself, who has denied Mr. Bolton’s account, circulated a derisive comment about the former national security adviser on Twitter, re-tweeting a message by Lou Dobbs, one of his favorite Fox News commentators, that slammed Mr. Bolton as a “Rejected Neocon.”

Senator Rand Paul, Republican of Kentucky — whose noninterventionist foreign policy views are in stark contrast with those of Mr. Bolton — questioned his credibility.

“You have an angry, disgruntled employee, and I think he’ll say anything,” Mr. Paul said.

Doug Stafford, Mr. Paul’s chief strategist, was even harsher.

“We told you not to hire neocon clown @AmbBolton,” he wrote in a tweet addressed to Mr. Trump. “NONE OF THE WARMONGERS ARE ON YOUR SIDE!”

Credit…Erin Schaff/The New York Times

One of the key prosecution witnesses at the center of President Trump’s impeachment trial on Sunday lectured Secretary of State Mike Pompeo — and, without directly saying so, Mr. Trump — about the importance of supporting Ukraine. William B. Taylor Jr., the former top American diplomat in Ukraine, took issue with Mr. Pompeo for reportedly questioning whether Americans care about that country. In a op-ed published in The New York Times, Mr. Taylor described the many reasons Ukraine, a country wedged between Russia and the countries of NATO, is vital to the United States.

“To support Ukraine is to support a rules-based international order that avoided war among major powers in Europe for seven decades,” he wrote, describing the many ways Russia attacks Ukraine and the West. “It is to support democracy over autocracy. It is to support freedom over unfreedom. Most Americans do.”

Mr. Taylor, who testified during the impeachment inquiry that the Trump administration waged a pressure campaign to get Ukraine to announce investigations into the president’s rivals, mostly steered clear of the impeachment case in his article. He said only that “no matter the outcome of the debate about the propriety of a phone call between the two presidents, the relationship between the United States and Ukraine is key to our national security.”

But Mr. Taylor’s rebuke of Mr. Pompeo is also a not-so-subtle reminder to the president and his Republican allies of the national security threat at the heart of the Democratic case: that by withholding security aid to gain political leverage, Mr. Trump made the country — and the Western world — less safe.

Credit…Calla Kessler/The New York Times

President Trump, who raged throughout the morning about the release of a White House memoir written by John Bolton, his former national security adviser, told reporters gathered at the White House that he had not actually seen the manuscript for the book.

Still, the president disputed as “false” two of the book’s major claims: that Mr. Bolton and others had tried to dissuade Mr. Trump from withholding military aide to Ukraine, and that the president was convinced that Ukraine had meddled in the 2016 election.

Credit…Doug Mills/The New York Times

Among the Senate Republicans angry at the White House on Monday over revelations about a manuscript submitted for review by John R. Bolton are top allies of President Trump, including Senator Mitch McConnell, the majority leader.

Just hours before the trial was set to resume, Mr. McConnell, Republican of Kentucky, and Senator Lindsey Graham, Republican of South Carolina, were privately pressing the president’s advisers for an explanation about Mr. Bolton’s account, which undercuts a key White House defense of the president and blindsided the senators, according to people familiar with their thinking.

Speaking to reporters in the basement of the Capitol on Monday, Mr. Graham declined to detail his talks with the White House and said the report would likely not change his position on the outcome of the trial. But asked if he trusted Mr. Bolton, Mr. Graham retorted: “I don’t know if I trust anybody right now.”

He said the former national security adviser “may be a relevant witness.”

Mr. Graham skipped a news conference in the Capitol Monday morning at which he was previously scheduled to appear.

Senators were told that White House lawyers planned to push ahead with their defense as planned on Monday, arguing that the president had not committed impeachable offenses.

Credit…Doug Mills/The New York Times

Senator Susan Collins, Republican of Maine and one of the most closely watched swing votes on calling witnesses, said in a statement on Monday that new revelations from John R. Bolton, the former White House national security adviser, “strengthen the case for witnesses” in President Trump’s impeachment trial.

Her comments came the morning after The New York Times reported that Mr. Bolton wrote in his forthcoming book that Mr. Trump had directly linked $391 million in military aid for Ukraine to investigations he wanted of his political rivals, saying that he would not release the money until he got the information he sought.

Ms. Collins, who has said that she is open to calling witnesses and would likely vote to do so, also appeared to suggest that other Republicans were now talking privately about whether to do the same.

“The reports about John Bolton’s book strengthen the case for witnesses and have prompted a number of conversations among my colleagues,” she said in the statement.

In the wake of weekend revelations from John R. Bolton, President Trump’s former national security adviser, Republican defenders of Mr. Trump are looking for any ammunition to undercut the Democratic argument in favor of calling new witnesses.

One example: a news conference from two decades ago by a current Democratic House manager arguing against the need for additional testimony.

Representative Jerrold Nadler, now the chairman of the House Judiciary Committee, was asked at a 1998 news conference whether Democrats defending then-President Bill Clinton from impeachment should support calling witnesses in the inquiry. Mr. Nadler said no.

“There are some people who think that we should ask to subpoena certain witnesses. My own personal opinion is that we shouldn’t,” Mr. Nadler told reporters. “My own personal opinion is that it’s the duty of those who want to impeach the president to make the case. They’ve got to call the witnesses who will establish the case. They haven’t done so.”

Republican senators have been arguing that Democrats in the House — not the Senate — were responsible for calling officials like him and others who were blocked by the president from testifying in the House inquiry. Now, as they look for ways to keep a handful of Republican moderates from jumping ship and voting with Democrats, they are hoping that comments, like the one from Mr. Nadler more than 20 years ago, might help.

Democrats don’t just want to hear from John R. Bolton, President Trump’s former national security adviser. They also want his notes — and getting them could help pave the way for other documents they have been demanding.

A report in The New York Times said that White House officials believe Mr. Bolton, who is writing a book, took notes that he should have left behind when he resigned his post. Representative Adam B. Schiff of California, the lead House manager, seized on that detail Monday morning, saying the notes may be more important for the Senate than seeing a copy of the draft manuscript of Mr. Bolton’s book.

“Now we learned that John Bolton took detailed notes and presumably these are contemporaneous,” Mr. Schiff said on CNN’s “New Day” program. “These notes took place while the events were happening, while they were fresh in his mind. Those, in many respects, are more important than the manuscript. So we ought to not only have John Bolton testify, but we ought to see what he wrote down in his notes at the time.”

In addition to calling for additional witnesses, Democrats have said the Senate should subpoena emails, text messages and other documents that Mr. Trump’s administration refused to hand over during the House impeachment inquiry. The existence of Mr. Bolton’s notes could further pressure Republicans to agree to seek the documents.

John R. Bolton was President Trump’s national security adviser for a year and a half, but he has been a fixture in conservative politics for a generation, making it difficult for Republicans to merely dismiss him as a partisan with an ax to grind. There is no better evidence of the complications his potential testimony would create for Senate Republicans than the campaign checks he has written many of them.

Mr. Bolton’s political action committee, named after himself, has already cut $10,000 checks this election cycle to three Republican senators who will have a vote on whether to subpoena him in the trial: Tom Cotton of Arkansas, Cory Gardner of Colorado and Thom Tillis of North Carolina. Mr. Gardner and Mr. Tillis are both facing tough re-election fights in swing states.

Mr. Bolton started the PAC in 2013 to advocate his interventionist foreign policy views, and it has given away more than $1 million.

Other Republican recipients currently in the Senate include Josh Hawley of Missouri, Roger Wicker of Mississippi, Richard M. Burr of North Carolina, Roy Blunt of Missouri, Charles E. Grassley of Iowa, Ron Johnson of Wisconsin and Marco Rubio of Florida.

Credit…Erin Schaff/The New York Times

For weeks, Senator Mitt Romney, Republican of Utah, has been the only Republican senator openly supportive of calling John R. Bolton, the former White House national security adviser, to testify in President Trump’s impeachment trial.

But on Monday, Mr. Romney predicted he may soon be less lonely, based on conversations he has had with Republican colleagues in the wake of the revelation that Mr. Bolton writes in a forthcoming book that Mr. Trump directly conditioned military aid for Ukraine on the country providing information about his political rivals.

“I think it’s increasingly likely that other Republicans will join those of us who think we should hear from John Bolton,” he told reporters in the Capitol.

Mr. Romney would not say which senators he was referring to, but said he had spoken with several in the wake of The Times report.

Credit…Anna Moneymaker/The New York Times

Pressure on Republican senators to call witnesses in President Trump’s impeachment trial is not only building in Washington. It is building at home.

On Monday, Majority Forward, an arm of a political action committee affiliated with Senate Democrats, announced it will be airing two new 30-second digital advertisements — one called “Oath,” and the other called “Rigged” — aimed at five Republican senators facing tough re-election battles: Martha McSally of Arizona; Cory Gardner of Colorado; Susan Collins of Maine; Joni Ernst of Iowa and Thom Tillis of North Carolina.

Republicans have so far been mostly silent in the wake of a damaging New York Times report about John Bolton, Mr. Trump’s former national security adviser, who is high on Democrats’ lists of potential witnesses and is writing a memoir scheduled for release in March.

“By refusing to get the facts and demand a fair trial from the onset, Senate Republicans are putting party politics over principle,” said J.B. Poersch, a former adviser to Senator Chuck Schumer, the minority leader, who is now president of Senate Majority Pac, which is financing the ad. “Our new ad campaign urges these vulnerable incumbents to do their jobs and demand a fair trial now.”

Mr. Bolton’s account that Mr. Trump told him he did not want to release $391 million in military aid to Ukraine until that country’s leaders investigated Democrats directly contradicts Mr. Trump’s assertion that there was no “quid pro quo” demand for investigations in exchange for the funding.

Revelations about a manuscript of John R. Bolton’s forthcoming book on his time as President Trump’s national security adviser have cranked up pressure on senators to subpoena him to testify in the impeachment trial. But they raise another natural question: Could the Democratic House simply subpoena Mr. Bolton itself if the Senate refuses?

Technically, yes — and it may come to that eventually if the Senate refuses.

But for now the House impeachment managers are refusing to entertain the idea, determined to keep pressure on senators to seek any relevant information as they are in the middle of a trial to decide whether to remove Mr. Trump from office.

“I don’t want to go into any kind of backup plan,” Representative Adam B. Schiff, Democrat of California and the lead impeachment manager, said Monday morning on CNN. “The reality is the senators ought to hear this witness firsthand.”

The reality is House Democrats know it would be far less valuable and potentially far more complicated for them to do the subpoenaing. Mr. Bolton said earlier this month that he would be willing to bypass the courts and quickly testify at the trial if subpoenaed. But when the House requested his appearance last fall during its impeachment inquiry and threatened a subpoena, his lawyer privately informed the committee that he would go to court rather than outright agree to testify — opening a legal fight that could have taken months or longer to sort out.

There is no guarantee Mr. Bolton would skip that legal fight to appear before the House if the Senate bypasses its opportunity to hear from him. Nor, Democrats know, would it even be as impactful if he did eventually testify in the House after a Senate acquittal is complete.

Such a hearing may produce terrible headlines for the president that could hurt him politically, but they would probably be discounted by the fact that the Senate would have already deemed his actions unworthy of removal from office.

Republicans also know this and have begun to suggest it is the House, not the Senate’s responsibility, to call Mr. Bolton. Even Mr. Trump had the idea Monday morning, misrepresenting the fact that the House did request Mr. Bolton’s testimony during its inquiry.

“The Democrat controlled House never even asked John Bolton to testify,” he incorrectly asserted on Twitter. “It is up to them, not up to the Senate!”

Credit…Anna Moneymaker/The New York Times

Republican defenders of President Trump on Monday redoubled their efforts to block any witnesses from being called in the impeachment trial in the wake of revelations from John R. Bolton, the president’s former national security adviser, that Mr. Trump directly linked Ukraine security aid to investigations of Democrats.

Senator Lindsey Graham, Republican of South Carolina and a staunch ally of Mr. Trump, warned on Twitter on Monday that if the Senate voted to consider witnesses, Republicans would insist on calling people who Mr. Trump considers “relevant” to the case, a list that includes former Vice President Joseph R. Biden Jr. and his son, Hunter Biden.

The tweet was in essence a renewal of a threat that Republicans have been using for weeks to try and discourage Democrats for pushing for witnesses. But it also underscored how Sunday’s news about Mr. Bolton may have increased the likelihood that the Senate will vote to hear from witnesses.

Before The New York Times’s report about the contents of Mr. Bolton’s manuscript, it appeared that there would not be enough Republican support to call witnesses. Mr. Graham’s warning suggests that Republicans may be less confident now, and are intensifying their efforts to do what they can to avoid it.

Just four Republican senators would have to vote with Democrats to subpoena witnesses like Mr. Bolton and other top administration officials. Republican Senators Mitt Romney of Utah and Susan Collins of Maine have already indicated they would support calling witnesses, and Mr. Romney has said he wants to hear from Mr. Bolton in particular.

Senators Lamar Alexander of Tennessee and Lisa Murkowski of Alaska, both of whom had previously signaled a potential openness to witnesses, more recently have suggested they might not vote to insist on them. But that was before the revelations about Mr. Bolton.

President Trump’s legal team’s second day of oral arguments in the Senate impeachment trial opens with fresh uncertainty on Monday, after The New York Times reported that the White House has for weeks had an unpublished manuscript by John R. Bolton, the former national security adviser, asserting that Mr. Trump refused to release military assistance for Ukraine until the country gave him investigative information about his political rivals.

By Monday morning, several Republican senators had angrily called the White House trying to determine who at the administration knew about Mr. Bolton’s manuscript, which aides there have had for several weeks, and what was in it. They told the White House they felt blindsided, according to people briefed on the calls who insisted on anonymity to describe private discussions.

One reason for their ire is that Mr. Bolton’s account flies in the face of the rationale the president’s lawyers have offered the Senate for his actions, and which many Republicans have latched onto themselves as a defense of his conduct.

For several days, Mr. Trump’s legal team’s defense of his hold on $391 million in aid earmarked for Ukraine has been that he never linked the freeze to his desire for investigations into Democrats, including former Vice President Joseph R. Biden Jr. Specifically, his team has zeroed in on the testimony of Gordon D. Sondland, the United States ambassador to the European Union, that he “presumed” the linkage, and that president cared more about an announcement than an actual investigation.

In the manuscript, Mr. Bolton writes that Mr. Trump told him in August that he didn’t want to free up the aid until Ukraine turned over Russia investigation materials related to Hillary Clinton, Mr. Trump’s opponent in 2016, and Mr. Biden, a leading candidate for the Democratic nomination this year.

That undercuts the claim by the deputy White House counsel, Michael Purpura, made in the well of the Senate on Saturday, that “not a single witness testified that the president himself said that there was any connection between any investigations and security assistance and a presidential meeting or anything else.”

While Mr. Bolton never testified, he has said he will testify if he receives a subpoena.

John Ullyot, a spokesman for the National Security Council, issued a carefully worded statement on Monday morning, 16 hours after the Times story was published.

“Ambassador Bolton’s manuscript was submitted to the N.S.C. for pre-publication review and has been under initial review by the N.S.C.,” he said. “No White House personnel outside N.S.C. have reviewed the manuscript.”

“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, referring to a widely debunked theory that the president had pursued about former Vice President Joseph R. Biden Jr. and his son Hunter.

In an unpublished manuscript of his upcoming book, Mr. Bolton described the White House decision to withhold military aid from Ukraine until he left the White House in September. As national security adviser, Mr. Bolton would have been involved in many of the high-level discussions about Ukraine.

More on Bolton’s Revelations

Credit…Doug Mills/The New York Times

The news of John R. Bolton’s account describing how President Trump tied aid for Ukraine to investigations into the Bidens, given in an unpublished book, first reported by The New York Times, could hardly come at a worse time for Mr. Trump, just as his lawyers have opened his defense on the Senate floor.

Senators will decide, probably by the end of the week, whether to call witnesses like Mr. Bolton. Until now, Mr. Trump seemed assured not only of acquittal but appeared likely to fend off the testimony of any more witnesses.

When Mr. Trump’s lawyers address the Senate Monday afternoon, they will face the challenge of explaining how his own former top aide says the president did exactly what they say he did not do — or trying to ignore it altogether.

The White House legal team will resume its opening arguments at 1 p.m. at the impeachment trial on Monday in dramatically new circumstances, with calls intensifying for the Senate to hear from witnesses after new revelations surfaced from John R. Bolton, President Trump’s former national security adviser.

The Times reported on Sunday that in drafts of an unpublished manuscript, Mr. Bolton recounts a conversation with the president in August in which Mr. Trump said he preferred not to unfreeze $391 million in security assistance to Ukraine unless officials there helped with investigations he sought into Democrats. That account directly contradicts two key components of the president’s impeachment defense: that the decision to freeze the aid was independent from his requests that Ukraine announce politically motivated investigations, and that Democrats had only “presumption” and hearsay to prove otherwise.

House Democratic managers said in a statement on Sunday night that “there can be no doubt now” that Mr. Bolton must be called as a witness. But the few Republicans who have indicated an openness to hearing from witnesses, such as Senator Susan Collins of Maine, had yet to respond to reports of the book. By Monday morning, some Republican senators had contacted the White House to inquire about who had visibility into the manuscript as the Senate trial unfolded a week earlier.

Mr. Trump pushed back on Mr. Bolton’s claims early Monday morning on Twitter, saying that his former adviser “never complained about this at the time of his very public termination,” and that Mr. Bolton was looking to sell books.

“I NEVER told John Bolton that the aid to Ukraine was tied to investigations into Democrats, including the Bidens,” Mr. Trump wrote just after midnight.

The White House lawyers will open their first extended day of arguments in the trial at 1 p.m., when they will expand on points they introduced in a brief two-hour session on Saturday. They have around 22 hours of time allotted to them on Monday and Tuesday to counter the House Democratic managers, but are not expected to use all of it.