DENVER, Colorado — Barack Obama demanded truth from the "real Mitt Romney" on Thursday as aides promised a "hard look" at strategy after the president's listless performance in the first White House debate.

Passionate, cutting and engaged, in stark comparison to his debate showing, Obama told 12,000 cheering supporters at a chilly lakeside rally in Denver that the Romney who turned up to their first head-to-head clash on Wednesday night was an impostor who had covered up unpopular positions.

"I met this very spirited fellow who claimed to be Mitt Romney," Obama said. "It couldn't have been Mitt Romney because the real Mitt Romney has been running around the country for the last year, promising $5 trillion in tax cuts for the wealthy.

"The fellow on stage last night said he didn't know anything about that. The real Mitt Romney said we don't need any more teachers in our schools. The fellow on stage last night -- he loves teachers, can't get enough of them.

"If you want to be president, you owe the American people the truth," a fired-up Obama told supporters anxious not to see him fritter away his opinion poll lead with less than five weeks to go before election day.

The president's feisty appearance represented a clear effort to shift the news cycle away from his stinking debate reviews and stall Romney's bid to create a comeback narrative following his unexpectedly strong showing.

Obama, who flew later to another rally in Wisconsin, seized on Romney's comment that he knew nothing about a tax break for companies that outsource jobs overseas, saying that if that was true, then the multi-millionaire former venture capitalist needed a new accountant.

"He seems to be doing just fine with his current accountant," he said, poking fun at Romney's complex offshore tax arrangements, which Democrats highlight to press the case he is indifferent to middle-class struggles.

About 40 million people watched Wednesday night's debate on television, according to preliminary figures by the Nielsen TV ratings service.

Romney basked in the plaudits for his performance as he addressed a fundraising event in Colorado before flying to Virginia, saying Americans had seen two contrasting visions for the future on stage in Denver.

The Republican challenger made a surprise appearance at the Conservative Political Action Conference, thrilling hundreds of attendees as he stepped on stage to join his sons who were scheduled guests.

"I know this is going to be a close-fought battle," Romney said. "We need to win Colorado. You know what, if we do, we are going to win back the White House."

In his first public appearance since trumping Obama in the first of three one-on-one debates, the former Massachusetts governor pursued attacks on what he portrayed as the president's continuing expansion of government.

"I saw the president's vision as trickle-down government and I don't think that's what America believes in," Romney said. "I see instead a prosperity that comes through freedom."

Obama turned in a lethargic and sometimes irritable performance, allowing a pumped-up Romney an opening in a White House race that had been trending away from the Republican after weeks of gaffes.

The president's campaign team signaled that it understood Obama's performance was below par and promised a rethink before the next clash in New York state in two weeks' time.

Top strategist David Axelrod told reporters on a damage limitation conference call that Romney's performance was laced with untruths that the campaign would now push back on.

"We are going to take a hard look at this," Axelrod said. "I'm sure we will make adjustments."

Independent fact checkers agreed that many of the more flagrant manipulations of the facts in Denver were committed by the Republican challenger.

"The fact checkers will have a field day on Romney," congressional expert Thomas Mann of the Brookings Institution think tank in Washington told AFP.

"He brazenly lied on numerous occasions and that could change the story line over the next several days."

The question now is whether Romney's debate performance has changed the minds of enough wavering voters in key swing states to give the Republican challenger a real chance on November 6.

Opinion polls in the coming days will provide the first clues.