TAMPA, Fla. — With Mitt Romney's selection of Representative Paul D. Ryan as his running mate, Florida quickly emerged on Monday as a critical test of the nationwide Republican gamble that concerns over the mounting federal debt can blunt potent Democratic attacks on conservative proposals to revamp Medicare.
As Mr. Romney campaigned through Florida on Monday, Democrats greeted him with a barrage of assaults, including a Web advertisement featuring worried elderly voters in this battleground state. The campaign took on a more heated air as President Obama suggested in Iowa that the Republican ticket would "end Medicare as we know it," a warning echoed in North Carolina by Vice President Joseph R. Biden Jr.
Assailing proposed changes to the retiree health plan is a time-tested line of attack, nowhere more so than here in Florida, where voters 65 and older made up 22 percent of the electorate in the 2008 presidential election. Polls show that a majority of elderly voters nationally oppose changes in Medicare or Social Security, which Mr. Ryan in the past has also proposed altering.
The implications extend beyond Florida. Elderly voters are significant forces in Iowa, New Hampshire, Ohio, and Virginia and Pennsylvania, all states that could help determine the outcome of the election.
Aides to Mr. Obama said they would focus on older voters in those states by spotlighting Mr. Ryan's proposal, broadly endorsed by Mr. Romney, to make Medicare a choice between private insurance and traditional coverage in the belief that more competition would drive down costs and improve care. Democrats say the plan, under which retirees would get a set amount of money from the government each year to purchase insurance coverage, would lead to higher costs and lower quality care for many retirees.
"I think it should be left alone," said Lee Berkowitz, 87, a Democrat in North Hollywood, Fla., who voted for Mr. Obama in 2008 but said he has not decided whom to support in November.
Mr. Romney and Mr. Ryan signaled that they intended to go on the offensive, challenging the assumption that Republicans were better off playing down the issue. They are gambling that anxiety about deficits, the influence of the Tea Party movement and changing demographics will give them a chance to convince voters that the time has come to confront the rapidly mounting costs of sustaining entitlement programs. Mr. Ryan is scheduled to visit Florida this month.
"Every even-numbered year in Florida, seniors are accustomed to Mediscare tactics; that's what Democrats do," said Ed Gillespie, a senior adviser to Mr. Romney. "The fact is, we're going to go on offense here. Because the president has raided the Medicare trust fund to the tune of $716 billion to pay for a massive expansion of government known as Obamacare."
"There won't be a single senior citizen in Florida who won't know that by November," he said.
Republican candidates have gained experience in campaigns where Democrats have focused on Mr. Ryan's Medicare proposal — in Congressional races in Nevada and New York — and have developed what they think is an effective way to counter it. That strategy includes assailing Mr. Obama's health care plan, and noting that it was paid for in part by taking over $700 billion from Medicare.
"The president's idea, for instance, for Medicare was to cut it by $700 billion," Mr. Romney said at a rally in St. Augustine. "That's not the right answer. We want to make sure we preserve and protect Medicare."
Key to a push-back, Republican officials said, is using elderly surrogates — like a candidate's parents — to counter the idea that the party's approach is heartless or would leave retirees worse off.
Mr. Ryan has already begun noting that his mother, a Medicare recipient, lives in southern Florida.
Mr. Romney's advisers argued that Mr. Ryan's proposal was nuanced enough, since it would not apply to anyone currently on, or soon to go on, Medicare, to partly blunt Mr. Obama's attacks.
For all that, even Republicans said the choice of Mr. Ryan had not made Mr. Romney's task in Florida any easier, particularly because he passed over Senator Marco Rubio, a Republican, for the slot. "Ryan Could Hurt Romney in Florida," read the banner headline on Sunday in The Miami Herald.
"It puts the state in play," said Joseph Gaylord, a Republican consultant who lives here. "Rubio would have been the best candidate."
And in Des Moines, on his first solo outing as a member of the ticket, Mr. Ryan was heckled on the issue as he tried to speak at the Iowa State Fair, an indication of the intensity of the battle ahead for him.
Still, Mr. Gaylord said: "It's a legitimate argument if it's an argument that goes unanswered. But you go on the offensive on it — the truth is, it's not going to affect any current seniors — and get that into every discussion about health care you have."
Democrats said the opening they saw on the issue was reflected in the aggressive way they had moved to hang Mr. Ryan's budget over Mr. Romney's candidacy.
"It's pretty powerful," said Representative Debbie Wasserman Schultz of Florida, who is also head of the Democratic National Committee. "We have the largest population of seniors in the country. If we go with the Romney-Ryan plan to end Medicare as we know it, you are going to jeopardize the lives of seniors, and you are going to jeopardize the Florida economy."
The key question, analysts said, is whether Mr. Ryan's argument about runaway federal spending and deficits would appeal to younger voters who might be worried that at the current trajectory, Medicare will soon become financially unsustainable or a giant burden on future generations.
"The truth is we simply cannot simply continue to pretend like a Medicare on track to go bankrupt at some point is acceptable," Mr. Romney said at a news conference in Miami. "We must take action to make sure that we can save Medicare for coming generations."
Brad Coker, the managing director of the Mason-Dixon poll, said Republicans might be helped by the unpopularity of Mr. Obama's health care law.
"This will be an interesting test," Mr. Coker said. "Medicare on its own has always been a powerful issue with the Democrats. This is the first time you test Medicare as an issue against the health care reform."
The elderly population in Florida is more diverse than it was in the days of Democratic snowbirds flocking here from Brooklyn and the Bronx: There are more people who have moved from states like Iowa and Georgia, and are more likely to be Republicans and, more significantly, affluent.
"The senior vote in Florida is a lot more complicated than it used to be," said Matthew T. Corrigan, a political scientist at the University of North Florida. "What Ryan does is he helps in places where you have really conservative seniors — the Tea Party seniors if you will."
Andrew Kohut, the director of the Pew Research Center, said national polls he had conducted had shown this was an issue of great concern for elderly voters. "If ever there was an issue that threatens the G.O.P.'s headlock on the senior vote, this is it," he said.
In interviews, some voters recoiled at the idea of changing the Medicare program immediately, though many said they were open to changes for future recipients.
"I'd be strongly opposed to the idea if it were to affect me," Phil English, 66, a retired high school teacher, said in Sun City, Ariz. "It's not being selfish, but we've worked for that, and we've planned for it."
In Pennsylvania, a state that Mr. Romney is hoping to put in play, Jennie Fiorenza, 93, a retired bookbinder, spoke warmly of a program that has become a critical part of her life.
"I think Medicare is wonderful, and I couldn't do without it," Ms. Fiorenza said.