Obama did jump on Romney's lack of specifics as the rivals clashed on taxes and health care reform, but the president stuttered through several of his more detailed answers, while his Republican opponent was crisper and clearer.

"The president has a view very similar to the view he had when he ran four years ago: that a bigger government, spending more, taxing more, regulating more - trickle-down government - would work," Romney said.

"That's not the right answer for America. I'll restore the vitality that gets America working again," he vowed.

Obama hit back by suggesting Romney would bring in US$5.4 trillion in tax cuts, particularly geared towards the wealthy, and said his Republican foe hadn't been clear on which loopholes in the tax system he would close.

Romney challenged the truth of Obama's claims as the tax issue sparked what proved the fiercest clashes in a low-key televised debate watched live by tens of millions of Americans.

"Virtually everything he said about my tax plan is inaccurate," the challenger said. "If the tax plan he described were a tax plan I was asked to support, I would say absolutely not."

Obama clings to a narrow lead in his bid to become only the second Democrat since the second world war to win a second term.

But Terry Madonna, director of the Centre for Politics and Public Affairs at Franklin and Marshall College in Lancaster, Pennsylvania, said: "I don't think there's any doubt Romney won.

"He was more aggressive without being pugnacious or provocative or combative. The president seemed a bit flat. He seemed, I wouldn't say annoyed at times, but almost disconnected, almost not comfortable."

The Romney campaign hailed a clear win for their man. "If this was a boxing match, it would have been called an hour into the fight," said top political adviser Eric Fehrnstrom.

The first debate focused strictly on economic issues. Foreign policy gets its turn in the last of the three presidential debates at the end of the month.

Romney, 65, badly needed to reset the election narrative, after a video emerged of him branding 47 per cent of Americans as people who pay no taxes and see themselves as "victims" who depend on government handouts.

Summing up, Romney said a re-elected Obama would usher in an era of falling household incomes and chronic joblessness.

"If the president is re-elected you will see a middle-class squeeze," he said. "You will see chronic unemployment - 43 straight months with unemployment above eight per cent."

Obama, who attacked Romney for having no plans - bar failed Republican policies that got the country into trouble in the first place - said he had lived up to his promises and asked US voters to give him another term.

"You know, four years ago, I said that I'm not a perfect man, and I wouldn't be a perfect president, and that's probably a promise Governor Romney probably thinks I've kept," he said.

"I also promised I would fight every single day on behalf of the American people, the middle class and all of those striving.

"I've kept that promise, and if you vote for me, then I promise I'll fight as hard in a second term."

Quotes of the night

Obama: "You know, four years ago, I said that I'm not a perfect man, and I wouldn't be a perfect president, and that's probably a promise Governor Romney probably thinks I've kept."

Romney: "Mr President, you are entitled to your own airplane, your own house as president, but not to your own facts."

Obama: "He now says he'll replace Obamacare and ensure all the good things in it will be in there. Is the reason that Governor Romney is keeping all these plans to replace secret because they are too good?"

Romney: "The president said he would cut the deficit in half. Unfortunately, he doubled it. Trillion dollar deficits for the last four years."

Obama: "Well, for 18 months he's been running on this tax plan, and now five weeks before the election, he says his big bold idea is 'never mind'."

Romney: "Spain spends 42 per cent of their total economy on government. We're now spending 42 per cent of our economy on government. I don't want to go down the path of Spain. I want to put more Americans to work."

Obama: "Now, the last point I would make, before ..."

Moderator: "Two minutes is up, sir."

Obama: "No, I think I had five seconds before you interrupted me."

Agence France-Presse