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Mr. Romney has given only broad outlines of his Medicare plan, which is less thorough than Mr. Ryan’s detailed budget.

“He doesn’t specifically say how all the details would work,” said Paul N. Van de Water, senior fellow at the liberal Center on Budget and Policy Priorities. “That leaves a lot of details unspecified.”

The Obama campaign said Mr. Romney’s new Medicare attack was false.

Mitt Romney’s position is Paul Ryan’s position,” Obama campaign spokeswoman Jen Psaki told reporters Tuesday. “They’re running on the same ticket, and their approach to Medicare and their approach to health care would be devastating for people across this country.”

When Mr. Romney picked Mr. Ryan to be his running mate, those on both sides of the aisle said it elevated this year’s election from back-and-forth squabbles to a broader debate about big ideas, such as how to reform entitlement programs and whether to raise taxes to pay for the promises the government has made to seniors over the decades.

But just a few days in, both sides have returned to the usual back-and-forth.

Democrats sound the alarm that Mr. Romney will end Medicare “as we know it,” while the GOP team argues that of those in the race, only Mr. Obama has succeeded in passing a law that takes money out of Medicare.

Mr. Romney has brushed aside lingering concerns that the election could turn into a referendum on Mr. Ryan’s spending blueprint.

“I have my budget plan, as you know, that I’ve put out. And that’s the budget plan that we’re going to run on,” Mr. Romney said during his first joint television interview with Mr. Ryan, on CBS’ “60 Minutes.”

Polls, though, show that seniors don’t like the idea of the federal government tinkering with their health care, and the Obama campaign is warning that the “Mitt Romney/Paul Ryan’s plan leaves the cost on the back of seniors” by fundamentally changing the way that the program is administered.