President Barack Obama mocked Mitt Romney's "two plus one equals five" economics, poking his Republican foe as he wrapped up his two-day trip across Florida in a bid to woo undecided voters.

Romney and his Republican running mate Paul Ryan meanwhile struggled to explain the accounting in their taxation plans, with fiscal policy and slow job creation taking centre stage in the White House race.

In Florida, Obama courted retirees by lambasting Romney's plans to reform the Medicare health care system for the elderly.

He also picked apart interviews in which the two Republicans declined to name any of the tax loopholes they plan to close to make their promise to cut taxes, while also trimming the deficit, add up.

"I guess my opponent has a plan but there is one thing missing from it -- arithmetic!" Obama said.

"It was like two plus one equals five. They couldn't answer questions about how they'd pay for $5-trillion in new tax cuts and $2-trillion in new defence spending without raising taxes on the middle class.

"That's not bold leadership - that's bad math. That gets a failing grade," said Obama, who had warned in an earlier interview with CBS News that the rich would have to pay more in taxes to alleviate the burden of the middle class.

In television appearances, Romney and Ryan declined to specify how exactly they would cut loopholes in the tax code to pay for rate reductions.

The Republican nominee, a multi-millionaire, also denied he planned to cut taxes for the rich.

"I want to make sure people understand, despite what the Democrats said at their convention, I am not reducing taxes on high-income taxpayers," Romney said on NBC's "Meet the Press."

As his armored bus trundled from west to east through Florida, Obama plunged into restaurants, diners and bars.

At one stop, he was surprised to be lifted a foot off the ground by hulking and exuberant pizza shop owner Scott Van Duzer.

"Look at that!" the president said once 113 kilograms, 1.90 metre, Van Duzer had deposited him back on terra firma.

"Man, are you a power lifter or what?"

On page two... Florida is a perennial swing state