Senator Joseph R. Biden Jr. proposed a compromise.

It was the fall of 2002 and the Bush administration was pushing for sweeping authority to act against Saddam Hussein, claiming he had weapons of mass destruction. Some Democrats questioned the stated threat posed by Iraq and bristled at President George W. Bush’s broad request.

Mr. Biden, the Senate Foreign Relations Committee chairman, had been scrambling to draft a bipartisan resolution that would grant Mr. Bush the authority to use military force against Iraq, but was more restrictive than the war authorization that the president had sought.

As he often had in his long career, Mr. Biden sought bipartisan middle ground — this time, between those opposed to potential war and the White House desire for more open-ended power. Some antiwar members of his committee resisted his effort, worried that it would still pave the way to conflict. “We disagreed very strenuously,” said former Senator Barbara Boxer, Democrat of California.

Mr. Biden’s plan ultimately did not succeed, and he chose to focus on Mr. Bush’s reassurances of a diplomacy-first approach.

“At each pivotal moment,” Mr. Biden said of Mr. Bush, “he has chosen a course of moderation and deliberation, and I believe he will continue to do so. At least that is my fervent hope.”

On Oct. 11, he was one of 77 senators to authorize the use of military force in Iraq. Twenty-three colleagues, some of whom harbored grave doubts about the danger Iraq posed at the time, refused to back the president’s request.

Nearly two decades later, Mr. Biden, who by 2005 was calling that vote a mistake, is running for president in part on his foreign policy experience, emphasizing his commander-in-chief credentials at a moment of heightened tensions between the United States and Iran.

Yet the Iraq war vote is part of the extensive record he cites, and he has struggled to accurately account for it on the campaign trail, repeatedly suggesting he opposed the war and Mr. Bush’s conduct from the beginning, claims that detailed fact checks have deemed wrong or misleading.

The vote has exposed him to direct and implicit criticism from his chief presidential rivals, including Senator Elizabeth Warren and former Mayor Pete Buttigieg, a military veteran, and especially Senator Bernie Sanders, who voted against the war as a Vermont congressman and whose campaign has sharpened its criticism of Mr. Biden in recent days.

Now, three weeks before the Iowa caucuses — held in a state with a fierce antiwar streak — the issue threatens to be a campaign liability for Mr. Biden as he seeks to assure voters of his ability to handle a foreign crisis even as he works to distance himself from a war that has had enormous costs for his own family, and for the nation.

A review of how Mr. Biden operated in the fall of 2002, as he weighed the question of authorizing the use of military force, reveals core truths about how he has worked for decades: as a Senate dealmaker at heart, with a reverence for bipartisan compromise that his supporters admire — and that critics say has colored his judgment during some of the most consequential moments of his career.

In the summer of 2002, as the Bush administration sounded alarms about Iraq, Mr. Biden, too, was concerned about the perceived threat of Mr. Hussein and his “relentless pursuit of weapons of mass destruction,” as he put it in a New York Times Op-Ed he wrote with Senator Richard G. Lugar, Republican of Indiana.

In the Dirksen Senate Office Building, Mr. Biden, a respected if long-winded chairman, convened two days of hearings about Iraq, soliciting testimony from a procession of experts.

“These weapons must be dislodged from Saddam Hussein,” Mr. Biden said, “or Saddam Hussein must be dislodged from power.”

But he also worried about the possible long-term consequences of confronting Mr. Hussein. “It would be a tragedy if we removed a tyrant in Iraq, only to leave chaos in its wake,” he said.

After Mr. Bush asked Congress in September to authorize the use of military force in Iraq, Mr. Biden and his colleagues grappled with how to proceed. Some Democrats balked that fall at the pressure to grant the president sweeping power, unmoved by Bush administration warnings of national security considerations, which turned out to be based in part on faulty intelligence and distortions of available information.

“Saddam Hussein is a threat, but the threat is not so great that we must be stampeded to provide such authority to this president just weeks before an election,” admonished Senator Robert C. Byrd, Democrat of West Virginia.

Mr. Biden was uneasy with the administration’s broad language and worried about the precedent pre-emptive action could set, but he was also focused on showing unity behind the commander in chief as the nation reeled from the terrorist attacks of Sept. 11, 2001. He hoped that, as he eventually said on the floor of the Senate, a “strong” congressional vote would ultimately give the United States a more forceful diplomatic position.

“We didn’t want to neuter the president because he’s our president, he conducts foreign policy, but at the same time we didn’t want to give him a blank check,” said former Senator Chuck Hagel, Republican of Nebraska, who worked closely with Mr. Biden.

Ever focused on bipartisan consensus, Mr. Biden joined forces with Mr. Lugar to produce alternative language that would give Mr. Bush narrower authority. The resolution authorized military action specifically for the purpose of dismantling a weapons of mass destruction program, and it emphasized “the importance of international support,” Mr. Biden and Mr. Lugar said at the time.

Some Democrats on Mr. Biden’s committee, alarmed by divergent intelligence assessments, were skeptical of his compromise effort.

“He tried hard to get us all together, he really did, he tried,” recalled Ms. Boxer, who detailed her concerns about the intelligence in her book, “The Art of Tough.” “But if there’s just a point where you feel the whole thing is based on false information, we couldn’t come on in good conscience.”

Their posture was deeply frustrating to Mr. Biden, a Senate institutionalist who was focused on vote counts.

“Does anybody here think the White House doesn’t have 55 votes for their resolution if we don’t have an alternative?” he railed at antiwar colleagues, according to his 2007 book, “Promises to Keep.” He added in the book, “I begged them to at least vote Biden-Lugar out of committee, but they made it clear they wouldn’t do it — on principle. They wanted purity.”

Reluctant Senate colleagues represented only one challenge that Mr. Biden faced. In a late-night phone call to Mr. Biden’s home, Mr. Lugar had informed him of a brewing counter-effort: The administration was working with Representative Richard A. Gephardt of Missouri, the House Democratic leader, on alternative language. Mr. Gephardt struck a deal with Mr. Bush and appeared with him and other lawmakers in the Rose Garden. (He was later critical of the Bush administration’s handling of Iraq.)

The deal had effectively torpedoed Mr. Biden’s proposal — and he seemed to know it.

“I’m sure the argument will be, well, why are we nit-picking?” he told reporters at the Capitol, clutching a coffee cup and, at one point, apparently exasperated, dismissing a reporter’s line of inquiry as “malarkey.”

The debate in Washington unfolded in a nation gripped by fears about terrorism.

“This country was really off balance and petrified and looking to the president to protect them,” Mr. Hagel said. “Members of Congress couldn’t get too far out politically to push back on the president, to say, ‘Well, I’m not sure that’s that important, I’m not sure he has weapons of mass destruction.’”

Politically ambitious Democrats were also leery of appearing weak on national security matters — and in the November 2002 midterm elections, Republicans would regain control of the Senate.

After his own proposal fell apart, Mr. Biden continued to meet with Bush administration officials. He found Secretary of State Colin L. Powell to be an influential and steady hand in the administration, and said publicly that the war resolution had been sufficiently improved from what the president had originally sought.

Nearly two decades later, the faith that Mr. Biden put in Mr. Bush is now a target for criticism.

“Many of his comments provided cover for proponents of the war — the war that many have come to realize was a fatal mistake,” said Jim Manley, who at the time was press secretary for Senator Edward M. Kennedy, Democrat of Massachusetts and a leading opponent of the war authorization.

On Saturday, the Sanders campaign released a blistering statement excoriating Mr. Biden for his Iraq war vote.

“Bernie Sanders saw the same information and had the judgment to vote against the Iraq war,” Jeff Weaver, a senior adviser to Mr. Sanders, said in the statement. Mr. Biden, he suggested, “undermined Democratic opposition, enthusiastically supported a disastrous war, refuses to admit mistakes, and then tries to rewrite history.”

Mr. Biden’s allies argue that his vote was intended to support the strengthening of diplomatic efforts in the hopes of preventing military conflict.

“For Biden and for a number of others who voted for the resolution, it was a vote for tough diplomacy,” said Antony J. Blinken, a longtime Biden adviser who served as Democratic staff director for the Senate Foreign Relations Committee. It is more likely that diplomacy will succeed, he argued, if the other side knows military action is possible.

As Mr. Biden eventually acknowledged, that did not work as he had hoped.

“It was a mistake to assume the president would use the authority we gave him properly,” he said on NBC’s “Meet the Press” in 2005.

But on the campaign trail this election cycle, he has suggested he opposed the war and Mr. Bush’s conduct from the beginning, claims that do not match the historical record.

“Immediately, the moment it started, I came out against the war at that moment,” he told NPR in an interview in September. His campaign later said he had misspoken, according to a fact check from The Washington Post. At a campaign stop in Des Moines this month, Mr. Biden said, “The president then went ahead with ‘shock and awe,’ and right after that, and from the very moment he did that, right after that, I opposed what he was doing,” a misleading assertion at best, according to an assessment from CNN.

Mr. Biden did ultimately become a vocal opponent of the Bush administration’s stewardship of the war, and went on to serve as vice president to Barack Obama, a critic of the conflict. The war took a personal toll when his elder son, Beau Biden, deployed to Iraq in 2008 with the Delaware Army National Guard. Beau Biden died in 2015 from brain cancer, and his father has discussed the possibility of a link between the illness and exposure to pits of burning waste on military bases.

Ms. Boxer attended a fund-raiser for Mr. Biden last week, though she said she was not yet formally endorsing him, and spoke warmly about her former Foreign Relations Committee colleague in an interview. She emphasized his record and all he had done in the nearly two decades since they clashed on Iraq.

“They fought very hard to get us on board and we fought very hard to get them to stop,” Ms. Boxer said. “Once he saw that it was a mistake, he really stepped up to the plate to try and come up with a way out of this war.”

Ahead of that 2002 vote, Mr. Biden stood on the Senate floor to explain his support for the war authorization. He followed Senator Hillary Clinton of New York — who in her later presidential campaigns also faced scrutiny over her Iraq war vote — and spoke for an hour.

“I do not believe this is a rush to war,” Mr. Biden said. “I believe it’s a march to peace and security.”

Hours later, he cast his vote in the affirmative.

Kitty Bennett contributed research.

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