At the June 3 debate in New Hampshire, for example, he was asked to defend his vote to continue financing the war in Iraq, a vote sought by the White House and criticized by fellow Democrats as an open-ended commitment to the war. All the other Democrats on stage voted against it, including Senator Barack Obama of Illinois, the presidential nominee who has picked Mr. Biden as his running mate.

“I love these guys who tell you they’re going to stop the war,” Mr. Biden said of his fellow Democrats. “Let me tell you straight up the truth. The truth of the matter is, the only one that’s emboldened the enemy has been George Bush by his policies, not us funding the war.”

One danger for Mr. Biden on Thursday is that his habit of speaking authoritatively, of saying he possesses the truth, will come across as overbearing or condescending, particularly toward someone like Ms. Palin, who lacks his credentials. To try to guard against sounding sexist, he is sparring in practice sessions with Gov. Jennifer M. Granholm of Michigan, who is playing the role of Ms. Palin.

The only other time a woman has appeared on the debate stage as part of a major-party ticket was in 1984, when Geraldine A. Ferraro, the Democratic vice-presidential nominee, faced Vice President George Bush. One exchange might offer Mr. Biden a good lesson.

Mr. Bush had said, “Let me help you with the difference, Mrs. Ferraro, between Iran and the embassy in Lebanon.” Ms. Ferraro instantly highlighted what she perceived as condescension: “I almost resent, Vice President Bush, your patronizing attitude that you have to teach me about foreign policy.”

Mr. Bush underscored one of the hazards of debating a woman when he later gloated into an open microphone, “We tried to kick a little ass last night.”

The risk may be even greater for Mr. Biden. His innate exuberance and gusto in speaking without stopping for air can make him sound like he is clubbing his points  and his opponent.