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.