Biden, the garrulous Senate Foreign Relations Committee chairman, is also at ease on camera and often showed his command of foreign policy issues and his flashing wit during a string of Democratic Party presidential primary debates in 2007.
But he has stumbled before. A debate gaffe ended his presidential hopes in August 1987, when he used an anecdote about a family hampered by lack of economic opportunity -- without crediting the source, British Labor Party leader Neil Kinnock.
For its part, Sen. John McCain's campaign appears to be taking no chances that Palin will prepare properly. It flew her Monday to McCain's Arizona ranch to cram with a coterie of the GOP presidential candidate's advisors.
As she began her run for governor of Alaska, Palin repeatedly proved difficult to prep for a debate, recalled her two former political aides, who had pivotal roles during her campaign but declined to be identified because of their continuing involvement in Alaska politics.
Palin, the former aides said, had a sharply limited attention span for absorbing the facts and policy angles required for all-topics debate preparation. Staffers were rarely able to get her to sit for more than half an hour of background work at a time before her concentration waned, hindered by cellphone calls and family affairs. "We were always fighting for her attention," said one of the aides.
In mid-October 2006, Palin's staffers saw their worries justified in the first political forum of the campaign season, an event at Anchorage's 49 Supper Club, where candidates unveiled their stump speeches before a room filled with political players. The former Wasilla mayor breezed through an upbeat speech about "taking back Alaska" but struggled during a question-and-answer session.
"To her credit, she gave a lot of 'I don't knows,' " one former aide recalled. "But it was clear she didn't start out with a great range of knowledge about Alaskan affairs."
In the weeks that followed, Palin's senior campaign aides took care not to let her repeat the dismal performance. "I was always frustrated because 30 minutes before game time, I'd want to say, 'Let's turn off the phone and lock the door. And please calm down,' " one of her former aides said.
But as time went on, Palin increasingly managed to zero in on the policy issues set before her during debate preparations, and her comfort level rose dramatically. During two final debates broadcast by Alaska public television and an Anchorage news station, Palin appeared to ace her performances, deftly crystallizing her talking points for voters.
