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When Mr. Romney picked Mr. Ryan, Medicare became a major issue in the campaign, and Mr. Biden is likely to do everything he can to score points on the topic during the debate.

Polls have given Mr. Obama the edge when it comes to handling Medicare. The most recent CNN survey had Mr. Obama leading Romney-Ryan on Medicare, 53 percent to 44 percent, and ABC News had Mr. Obama up 4 points on the issue.

In early September, while campaigning in Mr. Ryan’s home state of Wisconsin, Mr. Biden said the Romney-Ryan ticket would turn Medicare into “voucher-care.”

“We are for Medicare. They are for voucher care,” he said. “It’s basic.”

But Mr. Ryan has not tried to run away from the Medicare issue, memorably joined by his 78-year-old mother, Betty Douglas, when he delivered a speech on Medicare reform in The Villages, Fla., a large retirement community in mid-September.

During the address, he accused Mr. Obama of siphoning off hundreds of billions of dollars from Medicare to pay for his 2010 health care overhaul.

Medicare should not be a piggy bank for Obamacare,” he said, reassuring seniors that he is committed to protecting the Medicare guarantee for current seniors and for generations to come.

It’s a line Mr. Romney used repeatedly during last week’s debate against Mr. Obama, pointing out that Mr. Obama is responsible for shifting $716 billion out of Medicare to pay for his health care law.

But after the debate, Democrats said Mr. Ryan protected those cuts in his budget and fact-checkers have deemed the charge that Mr. Obama robbed Medicare of the amount as “mostly false” — a point Mr. Biden will undoubtedly try to hammer home Thursday night.