Biden gaffe on eve of presidential debate - NEWS.com.au

US Vice President Joe Biden inadvertently tossed raw meat to Republican rivals barely 24 hours ahead of the first presidential debate, saying the middle class has been "buried" during the last four years.

With President Barack Obama and Republican nominee Mitt Romney laying low ahead of Wednesday's Denver showdown, their running mates battled for the spotlight, and Republicans suggested Biden's gaffe marked a stunning admission five weeks away from the November 6 election.

"How they can justify raising taxes on the middle class that's been buried in the last four years," Biden, addressing supporters in North Carolina, said in reference to the period he and Obama have been leading the nation.

The White House quickly sought to douse the flames, saying Biden was talking about how president George W Bush's policies continued to hurt the middle class deep into Obama's term.

And Biden himself offered up his own correction in a tweet from his official Twitter feed: "'The middle class was buried by the policies that Romney and Ryan have supported."

But the Republicans, who argue that the middle class has been hard hit by four years of an Obama economy, let fly in the blink of an eye.

"Agree with @JoeBiden, the middle class has been buried the last 4 years, which is why we need a change in November," said a tweet from Mitt Romney's official Twitter account.

Romney's running mate Paul Ryan issued a scathing response.

"Unemployment has been above eight per cent for 43 months. Our economy is limping along right now. Vice President Biden, just today, said that the middle class, over the last four years, has been 'buried'. We agree," he told a rally in Iowa.

"That means we need to stop digging by electing Mitt Romney the next president of the United States."

Republicans suggested it would be an easy punchline for Romney during Wednesday's prime-time debate.

"Thank you Vice President Biden," former New Hampshire governor John Sununu, an aggressive Romney surrogate, quipped to reporters.

Meanwhile, the election protagonists were making final preparations for perhaps the high-profile moment of the 2012 campaign: a prime-time debate watched by tens of millions which could help determine the political future of the two rivals.

Obama will aim to maintain the aura of capable commander-in-chief who has steered America away from depression; Romney will strive to knock him off his pedestal on foreign policy and blame him for the stagnant economy.

Obama on Sunday downplayed his own debating skills, and said the clash should not be about who could fire off the best "zingers".

Romney, a multimillionaire investor and former governor of Massachusetts, offered a similar message, saying people should not focus on "who's going to score the punches", but on substance.

A Washington Post-ABC News poll on Monday gave Obama a slimmer 49 to 47 per cent lead, but, tellingly, likely voters in swing states sided with the president by 52 to 41 per cent.

And a CNN poll out on Tuesday showed Romney in a deep hole with Hispanic voters, who make up the country's largest ethnic minority demographic but who support Obama 70-26 per cent.