"Like a lot of Americans, when I think about Medicare it's not the program. It's not a bunch of numbers. It's what my mom relies on," Ryan told supporters in the key battleground state of Florida, home to a large number of retirees.

Ryan's 78-year-old mother, Betty Douglas, a part-time Florida resident, joined the candidate on stage at the rally in The Villages, a sprawling retirement community.

"My mom has been on Medicare for over ten years," Ryan said.

"She planned her retirement around this promise that the government made her," he argued, "and that's a promise we have to keep."

Florida is the largest of the so-called swing states expected to decide November's election, making its huge senior population a key demographic for both candidates.

President Barack Obama and his Democratic supporters have painted a withering picture of Ryan's plans for social programs, as laid out in the Wisconsin congressman's controversial budget proposals.

David Axelrod, a senior campaign advisor to Obama, recently called Ryan the "architect of a plan to end Medicare as we know it."

But Ryan defended his stance on Medicare, saying his plan would not affect benefits for seniors in or approaching retirement.

The 42-year-old claimed his changes would only have an impact on Americans under the age of 55, and that the changes were necessary to keep the program from going bankrupt.

Ryan's deficit-targeting plan called for deep cuts in US spending to deal with what Republicans say is an unsustainable US debt.

The plan included a proposal to introduce vouchers that seniors could use to purchase private health insurance.

Critics say the vouchers would be inadequate to cover the rising costs of health care, and that younger and healthier retirees would likely opt for private insurance, leaving only those with no other options -- the oldest, sickest and poorest -- to use the public program, in turn driving up its costs.

Former Massachusetts governor Mitt Romney, in the top slot on the Republican ticket, has already sought to put some distance between his running mate's proposals and his own.

"I have my budget plan... And that's the budget plan we're going to run on," Romney said in a recent interview with CBS News's "60 Minutes."