It's no surprise that Medicare has become a big campaign issue -- it is somewhat surprising that the Republicans are pushing it.

GOP presidential candidate Mitt Romney, running mate Paul Ryan, and other Republicans are stressing $716 billion in cuts to Medicare that are part of President Obama's health care plan.

That attack has forced Obama and company to play defense, even as they emphasize that Romney and Ryan want to turn Medicare into a voucher program that will cost seniors thousands of dollars a year.

The $716 billion in cuts are aimed not at Medicare recipients, but at health care providers, such as hospitals and medical device makers; they also target what the administration calls waste and inefficiency in Medicare.

"We made reforms that extended the life of the program," Obama said Saturday in New Hampshire, adding that his health care plan has reduced the costs of prescription drugs and provides preventive services like cancer screenings.

As for Republican idea of Medicare vouchers -- to be used in the private insurance market -- Obama cited studies estimating "that this could force seniors to pay as much as an extra $6,400 a year for their health care."

Meanwhile, speaking in Florida -- where the Medicare issue is particularly resonant -- Ryan told a group of senior citizens that Obama's plan "raids $716 billion from the Medicare program to pay for the Obama care program."

Ryan said the cuts are hurting nursing homes and Medicare Advantage insurance plans, and that "Medicare should not be used as a piggy bank for Obama care."

The Republican plan would not affect Americans who are already 55 or over, say Ryan, Romney and other Republicans. But changes must be made for future beneficiaries or the program will go insolvent, they say.

Speaking to a crowd that included his 78-year-old mother, Ryan said: "Mitt Romney and I will protect and strengthen Medicare so that the promises that were made that people organize their retirements around, like my mom, will be promises that are kept."

Obama aides quickly point out that Ryan included the $716 billion in Medicare reductions in his own budget, though Romney has not; the Republican candidate has vowed to repeal Obamacare in its entirety.

Republicans, who many analysts consider to be vulnerable on the Medicare issue, say they welcome the debate over the program's future. Said Ryan: "We want this debate, we need this debate, and we are going to win this debate."

Obama and aides are equally confident about the Medicare issue.

"My plan has extended the life of Medicare by nearly a decade," Obama said. "Their plan would shorten the life of Medicare and end Medicare as we know it, because they'd turn it into a voucher system."