Mitt Romney seized upon President Barack Obama's admission Thursday that he "can't change Washington from the inside" to brand him a failure, as the White House bout heats up.

With polls showing the incumbent enjoying a slim lead over his rival ahead of the November 6 vote, Romney leaned in with an aggressive thrust during a rowdy campaign stop in the battleground state of Florida.

"The president today threw in the white flag of surrender again. He said he can't change Washington from the inside, he can only change it from outside," Romney told supporters.

"Well we're going to give him that chance in November. He's going outside!" Romney said, to a roar from the 4,000-strong crowd packed into a museum's grassy courtyard under a blazing Florida sun.

The former Massachusetts governor had pounced on comments made earlier in the day by Obama, who was also in the Sunshine State for an appearance on Spanish-language network Univision.

"I think that I've learned some lessons over the last four years. And the most important lesson I've learned is that you can't change Washington from the inside. You can only change it from the outside," Obama said.

"That's how I got elected, and that's how the big accomplishments, like health care, got done ... because we mobilized the American people to speak out."

Romney's retort came amid a difficult and potentially pivotal week for the Republican challenger.

Battered by relentless criticism over remarks disparaging around half of the American population that emerged in a secret video, the Republican nominee is struggling to get his campaign back on track.

Another setback came early Thursday, when Romney's campaign co-chairman Tim Pawlenty, himself a 2012 White House hopeful before dropping out early, stepped down in order to head up a banking lobby.

The former Minnesota governor has been a fixture on Romney's campaign trail, and was seen as a likely contender to be Romney's running mate.

Budget hawk congressman Paul Ryan got the nod instead, and Pawlenty now heads to Washington to become a banking lobbyist.

Despite aides insisting the race is a dead heat, particularly in around nine battleground states that will ultimately decide the election, Obama has a consistent if narrow lead in most polls.

Romney's rally was near the "I-4 corridor" that crosses the heart of the Florida and is seen as one of the most vital areas of the country to secure key independent or undecided voters.

Standing before a large banner that read "Protect and Strengthen Medicare," Romney pledged to thousands of seniors in attendance that he would do away with Obama's landmark health care reform law.

A day earlier in Miami, on the same "Meet the Candidates" program addressed by Obama, Romney had muddled his message by quipping that he didn't mind when the president called him "the grandfather of Obamacare."

"I don't think he meant that as a compliment, but I'll take it," Romney said.

As governor of Massachusetts Romney signed major health care into law, including a mandate obliging citizens to obtain health insurance or pay a fine, a key feature of Obama's plan.

Romney spent much of Wednesday embracing the anti-dependency platform he had espoused in the secret video footage, telling donors in Atlanta, Georgia that Obama was pushing a more socialist-styled role for government.

America "does not work by a government saying 'become dependent on government, become dependent upon redistribution.' That will kill the American entrepreneurship that's lifted our economy over the years," Romney said.

But in Miami he added an element of inclusiveness and unity to his message.

"My campaign is about the 100 percent of America," Romney said.

Romney and Ryan embark Monday on a three-day bus tour of Ohio, the Midwestern battleground state that is critical to the Republican challenger's hopes of victory.