President Barack Obama told voters in Florida on Thursday that Republican challenger Mitt Romney wants to undo the structures of American society that built a vibrant middle-class.

Mr. Romney, speaking in a Boston suburb, repeated his charge that Mr. Obama was more concerned with his re-election than bringing down the nation’s high rate of unemployment.

The men battling to win the November election for the White House are engaging in the kind of intense campaigning normally reserved for the final weeks of the contest.

Mr. Obama’s decision to spend two days in Florida signals the importance of the state. If the President can win there, as he did in 2008, Mr. Romney would have a difficult time blocking the President’s return to the White House.

The race promises to be even closer in Florida. The state provided the deciding margin in George W. Bush’s victory in 2000, and it has been closely contested ever since. Mr. Obama aides said that since 1992, voters in the state have cast more than 32.5 million votes during the past five presidential elections, and only a total of 57,000 votes have separated the two parties in those campaigns.

Republicans are holding their national convention in Florida in August in hopes of giving themselves an edge in the state.

Mr. Obama was hitting hard on themes important to residents of Florida, a state heavily populated with retired Americans, military personnel and a middle-class worried about the slow economic recovery and unemployment.

Mr. Obama said the basic bargain in the United States was “at risk like never before” because of Republican policies, those promoted by Mr. Romney who has a fortune estimated at a quarter billion dollars.

“No matter who you are, no matter where you come from, no matter what you look like, American has always been a place where you can make it if you work hard,” Mr. Obama said.

Mr. Obama, speaking directly to retired Americans, hammered Mr. Romney for backing a plan that would overhaul Medicare, government health insurance for those over 65. He said those changes would end up costing retired people an additional $6,400 a year.

As Mr. Obama spoke in Florida, Mr. Romney appeared at a hastily-arranged event in a suburb of Boston, headquarters for his campaign.

“The job he’s interested in protecting is his own,” Mr. Romney said. “The job that should be protected is the job of men and women in this country who want to get jobs, who want to go back to work, who want to have rising incomes again.”

“The President does, in fact, believe that people who build enterprises like this really aren’t responsible for it,” Mr. Romney said at Middlesex Truck & Coach.