Aides joked that the sparring partners spent so much time together this week that they were even starting to dress alike. Both men showed up to debate practice Wednesday in blazers and blue dress shirts.
The 90-minute debate is sure to draw a television audience of tens of millions. But it's unlikely to eclipse the 70 million who tuned in to watch Biden face off with Republican firebrand Sarah Palin four years ago.
That debate was more of a curiosity: It allowed Palin to outdo Biden in folksiness and recover from a series of painfully awkward media interviews but did little to alter the trajectory of the race.
"Joe just needs to be Joe," Obama said, when asked his advice for the vice president in an interview Wednesday with ABC News.
Thursday was a rare day when the political activities of the running mates were taking center stage and those of Obama and Romney were seen as secondary. But with just 26 days left until the election and the race still tight, neither Obama nor Romney was completely ceding the spotlight, with each holding a rally in a swing state.
Obama told the Florida rally that Romney is changing his positions after winning the Republican nomination to appeal to general election voters. "After running for more than a year in which he called himself severely conservative, Mitt Romney is trying to convince you that he was severely kidding," the president said to laughter from his supporters.
Obama also was attending a fundraiser in Miami featuring actress Eva Longoria and singer Sheila E. About 700 people were expected, with tickets costing between $500 and $10,000 per person. Obama campaign spokeswoman Jen Psaki said the fundraiser would be one of the final ones of the campaign for the president.
Thursday's debate, moderated by Martha Raddatz of ABC News, will cover both foreign and domestic topics. The debate is to be divided into 10 segments. At the outset of each segment, Raddatz will ask an opening question, and each candidate will have two minutes to respond.
Both Biden and Ryan head into the debate with vulnerabilities: Biden must rein in a freewheeling manner that can be endearing but also produces plenty of gaffes. Ryan hasn't been in a campaign debate for more than a decade and is light on foreign policy experience, a sharp contrast to the vice president, a former chairman of the Senate Foreign Relations Committee.
Ryan also will need to find a way to reinforce Romney's policy positions without selling out his own, more conservative credentials.
Associated Press writers Julie Pace and Nancy Benac in Washington, Ken Thomas in Coral Gables, Fla., and Robert Ray in Wilmington, Del., contributed to this report.
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