Biden said he has been "studying up" on Ryan's positions to prepare. "I just want to make sure that when I say these things that I don't have the congressman say 'No, no, no, I don't have that position,' " Biden said.
"We have a fundamentally different view on a whole range of issues," he said.
Biden has been closeted in his hometown of Wilmington, Del., since Saturday for practice sessions with Van Hollen and prep work with his staff and top advisers to Obama, including senior campaign strategist David Axelrod. Biden has participated in mock debates and studied videos of Ryan's speeches and interviews.
Ted Kaufman, a longtime staffer who replaced Biden in the Senate after he became vice president, said Biden is comfortable in debates, but faces a crafty opponent in Ryan.
"The problem with this one is you just don't know what Congressman Ryan is going to say," Kaufman said, noting what he called Romney's repeated disavowals of his own proposals on taxes and others issues during last week's debate. Democrats expect the same from Ryan, Kaufman said.
At his acceptance speech at the Republican National Convention, "Ryan said a bunch of stuff that wasn't true," Kaufman said, citing Ryan's claim that Obama had failed to deliver on a promise to save an auto plant in Ryan's hometown of Janesville, Wis. The plant closed before Obama took office.
Ryan also faulted Obama for a proposed $716 billion Medicare cut that Ryan embraced in his own budget.
Despite those concerns, Kaufman said his advice to Biden is to focus less on his opponent than on his own performance. "I think the best thing you can do in one of these things is to be yourself," he said.
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