Mr. Biden supported the 2002 Iraq war resolution and thus is at odds with Mr. Obama, who opposed the war from the start and has made his judgment on the question a centerpiece of his campaign. But Mr. Biden tempered his support for the conflict by saying it should be limited to ending Iraq’s weapons programs, and he has been a sharp and persistent critic of the war’s conduct.

Last year, he said giving the Bush administration the authority to wage war was a mistake. “I regret my vote,” he told Politico, the politics Web site. “The president did not level with us.”

Mr. Biden’s appeal as a national candidate is suspect. His first bid for the presidency, beginning in 1987, famously flamed out after he was caught stealing passages from a speech by Neil Kinnock, the leader of the Labor Party in Britain at the time. His 2008 campaign never really got off the ground, little noticed in a field of well-financed candidates. He finished fifth in the Iowa caucuses, edging out “uncommitted.” He promptly dropped out and, diplomatically, declined to take sides in the slugfest between his Senate colleagues Mr. Obama and Hillary Rodham Clinton.

After the primaries ended, he enthusiastically endorsed Mr. Obama, though Mr. Biden had offended many around Mr. Obama early in 2007 by describing him as “the first mainstream African-American who is articulate and bright and clean and a nice-looking guy.” If there were any hard feelings, they appear to have been forgotten.

Mr. Biden, normally among the most accessible of senators, declined to be interviewed for this article.

Despite his poor showing in the presidential race, Mr. Biden was generally thought to have acquitted himself well. His debate performances were uncharacteristically crisp, and he delivered two of the more memorable lines of the campaign.

Asked in an April 2007 debate whether he had the self-discipline to lead the free world, Mr. Biden answered, “Yes.”