(CNN) - In their first joint interview as the 2012 Republican ticket, Mitt Romney and Rep. Paul Ryan on Sunday addressed questions about Ryan's Medicare proposal, which at least one critic says would "end Medicare as we know it" and others have speculated could present Romney electoral challenges in senior citizen-rich Florida.
Ryan, however, has said major changes are necessary to rein in levels of federal spending on the program which premium payments and payroll taxes cannot cover. He used a personal example to make the point in his Sunday interview.
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"My mom is a Medicare senior in Florida," he said on CBS. "Our point is we need to preserve their benefits, because government made promises to them that they've organized their retirements around."
"In order to make sure we can do that, you must reform it for those of us who are younger. And we think these reforms are good reforms. That have bipartisan origins. They started from the Clinton commission in the late '90s," he continued.
On Sunday, Romney campaign aide Brendan Buck said Ryan would campaign in central Florida next weekend, a rebuff to critics who said the vice presidential candidate was skipping Romney's stops in Florida this week to avoid questions about his Medicare proposals from senior citizens there.
Romney has not made Ryan's plan a part of his platform in the 2012 campaign, and in a set of campaign talking points obtained by CNN on Saturday, surrogates are reminded, "Of course they aren't going to have the same view on every issue."
A suggested answer to questions about how Ryan will play among independent voters read "Mitt Romney and Paul Ryan have a bipartisan plan to strengthen Medicare by giving future seniors the choice between traditional Medicare and a variety of private plans."
In the interview, Romney turned the Medicare question into a criticism of President Barack Obama, repeating a line he uses on the campaign trail.
"There's only one president that I know of in history that robbed Medicare, $716 billion to pay for a new risky program of his own that we call Obamacare," he said. "What Paul Ryan and I have talked about is saving Medicare, is providing people greater choice in Medicare, making sure it's there for current seniors."
The $716 billion figure appears to come from a Congressional Budget Office evaluation from July 24, which measured the impact of repealing the health care overhaul known as "Obamacare."
The report says that under the repeal measure, "Spending for Medicare would increase by an estimated $716 billion over that 2013-2022 period."
Those spending increases would be a result of more spending on hospital and medical insurance, offset by a decrease in prescription drug coverage.
The letter also notes that the projected $716 billion increase in Medicare spending if the measure is repealed does not signal a $716 billion decrease if the measure stays in place - which is Romney's argument.
"For various reasons discussed elsewhere in this document, the estimated budgetary effects of repealing the ACA...are not equivalent to an estimate of the budgetary effects of the ACA with the signs reversed," the report says.
Romney, in his remarks, stressed that his plan would ensure "No changes, by the way, for current seniors, or those nearing retirement."
"But looking for young people down the road and saying, 'We're going to give you a bigger choice.' In America, the nature of this country has been giving people more freedom, more choices. That's how we make Medicare work down the road," he continued.
Romney announced Ryan, chairman of the House Budget Committee and a seven-term member of Congress from Wisconsin, as his vice presidential pick on Saturday at a campaign rally in Virginia.
CNN's Peter Hamby contributed to this report
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