DENVER: Mitt Romney tore into Wednesday's first presidential debate with the driving passion of a man who thinks he is losing an election.

President Barack Obama appeared every inch the battered incumbent, drained by four painful years, apparently playing it safe because, until Wednesday at least, he was winning.

But at the end of 90 minutes of thrust and parry it was Obama's team which was thrown on the defensive, as Romney seemed to carry off the victory he needed to fire up what had been a flagging campaign.

The president's team apparently believes that the Republican's smooth performance will soon show up in polling results ahead of the November 6 election.

“We don't care about national polls,” Obama senior advisor David Plouffe told reporters, arguing that the complicated state-by-state election map remains problematic for Romney.

“The question is, in those battleground states, what is Romney's path to 270?” Plouffe said, referring to the electoral vote total needed to win the White House.

US presidents live in rarified air in their White House bubble, and are rarely contradicted or directly challenged.

That was clear to see on Wednesday night, as Obama appeared irritated as Romney launched a searing attack on his record.

“But you've been president for four years,” Romney said, in a spirited clash over the deficit, as the Republican relished the chance to try to turn the election into a referendum on Obama's record and not his own liabilities.

There was none of the fire or the passion that pulsated through Obama's campaign in 2008 but has been a more fleeting feature of his re-election race.

On a thick red carpet stretched over an ice rink at the University of Denver, the challenger appeared to get the best of the argument, coming across as more energetic, full of ideas and passionate.

First impressions after a debate are often deceptive, and the media narrative usually hardens only after a few days.

Yet Romney won immediate praise, and Obama appeared to miss several attempts to disqualify the Republican as a potential president.

Obama's senior aides were left complaining that Romney went overboard in his attacks -- a sure sign of a campaign that thinks it lost the debate.

“What you saw tonight was what we saw during the primaries, which is Governor Romney is a very eager and willing candidate on the attack,” said Obama's friend and senior advisor David Axelrod. “And that's what he did tonight. You saw his closing statement was nothing but unalloyed carpet bombing.”

Romney picked apart Obama's economic policies, speaking confidently, smoothly and in well crafted soundbites, emboldened by standing side-by-side with the man he has been lambasting on the campaign trail for four years.

By contrast, Obama appeared to be on the defensive for large portions of the 90-minute clash, appearing less energetic than his challenger, despite the fact that that he is 14 years younger.

Obama's debate parries were torn from his regular stump speech, and there was very little new material.

Aides beforehand expressed fears that the president, who has trudged through successive crises in the White House after the worst recession since the 1930s, was out of practice in debates.

That scenario appeared to play out, as Obama failed to cross-examine Romney's claims about his own policies and critiques of the president.

In a debate about the economy, he never mentioned his claim to have created five million jobs, and did not highlight the auto bailout that he sees as one of his main accomplishments.

Strangely, he made no reference, or even an oblique jibe, at Romney's most flagrant missteps, including the secret tape in which the Republican branded 47 percent of Americans as “victims” who dodge taxes.

The president also did not mention the refusal by Romney, a multi-millionaire former investment banker, to release more than two years of tax returns in a departure from tradition for candidates.

Snap polls by US television networks all had Romney winning the debate by a large margin, with CBS News, for example, finding that 46 percent of undecided voters said Romney won, while only 22 percent picked Obama.

Obama's apparent passivity was a stark contrast to the harshly negative attacks his campaign has made on Romney's business background and character.

Apparently, the Obama team had decided to offer specifics and avoid getting drawn into a fierce fight.

The president took a similar approach in his convention speech last month, which was panned by critics but helped him pull out a lead in national polls and in key battleground states.