MIAMI — US President Barack Obama quipped Thursday that Mitt Romney must be "severely kidding" if he thought Americans would fall for his sudden "makeover" designed to hide extreme conservatism.

Obama, shocked into action by poll numbers driven down by his sleepy debate performance last week, ripped into his Republican foe in swing state Florida, possibly hinting at his tactics at next week's second head-to-head.

"He's trying to go through an extreme makeover. After running for more than a year in which he called himself severely conservative, Mitt Romney's trying to convince you that he was severely kidding," Obama told a 9,200-strong crowd.

Obama's passionate attack fleshed out a new theme adopted by his campaign, namely that Romney, after running to the right to win the Republican nomination, is covering up hardcore conservative stands to woo moderates.

The president teed off on the "latest version of Mr Romney" and again claimed his foe was advancing a $5 trillion tax cut and claiming it would pay for itself, likening his approach to deficit-busting George W. Bush economics.

"We heard that same argument back in 2000, back in 2001, back in 2003. We have heard this pitch before -- we know it doesn't work. We know our plan does," Obama roared to a crowd made up mostly of students.

Obama also struck a fresh note by warning that if elected, Romney would do nothing to moderate the ideas of Republicans who are expected to retain the House of Representatives and are challenging for control of the Senate.

"We know full well that if he gets a chance, Governor Romney will rubber stamp the top-down agenda of this Republican Congress the second he takes office and we cannot afford that future," Obama said.

"His plan will not create jobs. It will not help the middle class. It will not speed the recovery. It will slow down the recovery. It will not reduce the deficit. It will not expand opportunity.

"We can't afford it. We're not going back. We are moving forward."

Obama has made increasingly obvious efforts to inject more urgency, passion and immediacy into his public comments after the debate last week in Denver, which Romney clearly won, triggering a surge in the polls.

In an NBC/Wall Street Journal poll in Florida, the largest swing state, released Thursday, Obama led Romney by a single point, down from a five-point lead a month ago.

In national polls, the race is essentially a dead heat ahead of the November 6 election and Romney has closed the gap, or even overtaken Obama in some polls of the battleground states that will decide the election.

Obama, who addressed an overflow room of supporters in Florida after his speech, is trying to drive up the vote among students, a key plank of his coalition, less than four weeks from election day.

He had a second, fundraising event planned in Miami on Thursday evening before a return flight to Washington.

Aides said the president would watch Joe Biden try to stem Republican momentum in his debate clash with Romney's running mate Paul Ryan, later on Thursday on television on the iconic blue and white jet.

Obama will then travel to another swing state, Virginia, on Saturday, for an intensive debate camp, ahead of his second, and crucial clash with Romney on Tuesday in New York state.