Both candidates were locked in debate preparations for three days, Palin with her husband and children at McCain's vacation compound near Sedona, Ariz., and Biden in Wilmington, Del.

Palin, who has traveled with a briefing team since Sept. 10, practiced with Randy Scheunemann, McCain's chief foreign policy aide; Steve Beigun, a former staff member of President Bush's National Security Council; Nicolle Wallace, a top McCain adviser and a communications director in the Bush White House; and Mark Wallace, a deputy manager of Bush's 2004 re-election campaign, who is Nicolle Wallace's husband.

People close to the McCain campaign have said that the team initially bombarded Palin with briefing books and facts as if preparing her for a Ph.D. oral examination but eased up after her interviews with Couric, in which Palin appeared to be stringing together memorized talking points, at times in nonsensical fashion. Although McCain defended her, a number of influential conservatives said Palin's answers amounted to gibberish.

Biden's preparations were rooted far more in style than substance, aides said, as he ran through hours of practice sessions with a goal of keeping his answers tight, holding his focus and avoiding saying anything that could be interpreted as offensive toward his rival. He also watched Palin's debates from the campaign for Alaska governor, as well as some of his own debates from the Democratic primary.

In Biden's practice sessions, Gov. Jennifer M. Granholm of Michigan played the role of Palin; Biden also received guidance and suggestions from Clinton and Sen. Dianne Feinstein of California. Campaign officials dispatched virtually all of Obama's senior female advisers to St. Louis to lead the surrogate outreach before and after the event.

The debate was predicted to have a huge audience, with the potential to break the record for the most-watched vice presidential debate, the 1984 matchup between Geraldine Ferraro and the elder George Bush, which drew 56.7 million viewers.

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Contributing: Kate Zernike