But critics see a massive cost shift to beneficiaries.
"The only way to drive real savings is to set a lid on the growth in the voucher," said Democratic economist Judy Feder. "That most likely means shifting costs to beneficiaries, not controlling costs."
In an analysis earlier this year, the nonpartisan Congressional Budget Office said some of the effects of Ryan's plan "would of necessity be a great deal stronger" than current law, which includes Medicare cuts in Obama's health care law yet to take place.
Under the most likely current budget scenario, Medicare spending for the typical 66-year-old would rise to $9,600 in 2030, or about 75 percent more than now, the CBO projected.
But under Ryan's plan, spending would rise more slowly to $7,400, or about 35 percent more than current levels.
That difference would result in a cost shift of thousands of dollars to individual retirees, critics say.
Under the previous version of Ryan's plan, a typical 65-year-old retiree would have been responsible for about two-thirds of his or her health care costs in 2030, according to the budget office. That translates to a cost increase of $6,350 a year, says the Obama campaign.
The political sensitivities are clear. Polls find that Americans lean heavily on Medicare to help keep them secure after retirement and are suspicious of proposed alternatives, such as Ryan's. Surveys also give Democrats an edge over Republicans when people are asked which party people most trust to handle Medicare. Democrats held a 48 percent-39 percent advantage on that issue in a June 2011 AP-GfK poll.
"This puts Medicare in play as a central issue in the campaign," said John Rother, president of the National Coalition on Health Care, a nonpartisan group representing a broad swath of players in the health care system.