With President Barack Obama and Republican nominee Mitt Romney laying low the day before the Denver showdown, the spotlight shifted to the running mates and Republicans suggested Biden's gaffe marked a stunning admission five weeks away from the November 6 election.
Speaking to supporters in North Carolina about the Romney campaign's tax plan, Biden asked "How they can justify raising taxes on the middle class that's been buried in the last four years?"
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.
Biden himself offered a correction on 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 Romney's official Twitter account.
His running mate Paul Ryan also piled on.
"Unemployment has been above 8% 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," Ryan 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 the prime-time debate, but Obama's team warned that the multimillionaire would likely have some explaining to do about whether his offshore holdings were set up to avoid taxes.
On the eve of the clash in Denver, a New York Times report suggested tax arrangements of Bain Capital, the private equity firm Romney founded, may have helped enhance his wealth.
"This raises a lot of questions that the Romney campaign should have to answer" on Wednesday, said Obama spokesperson Jen Psaki.
"We look forward to hearing what he has to say."
The Times examined thousands of pages of previously unreported Bain documents relating to dozens of offshore holdings in the Cayman Islands.
The paper said the arrangements in some cases allowed the former venture capitalist's retirement account to avoid taxes and may have cut his personal income tax bills.
Meanwhile the election protagonists were making final preparations for perhaps the most 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 a capable commander-in-chief who has steered America away from economic depression; Romney will strive to knock him off his pedestal on foreign policy and blame him for the stagnant economy.
The Republican challenger spent the day with top aides and Ohio Senator Rob Portman, who is playing Obama in mock debates. When the nominee ducked out for lunch at a Denver restaurant, reporters asked if he was ready.
"I'm getting there," Romney said.
Obama, too, took a break from debate camp to tour the Hoover Dam, the vast concrete bohemoth on the Colorado River that is a symbol of public works projects undertaken in the aftermath of the 1930s Great Depression.
"It's spectacular, and I've never seen it before," Obama said, ignoring questions by reporters about how his debate practice was going.
Meanwhile debate organisers were putting the finishing touches on the venue, where a huge plaque depicting an American bald eagle hangs above the stage, bearing a banner reading "The Union and the Constitution Forever."
Janet Brown, executive director of the Commission on Presidential Debates, said that come Wednesday, the 1 000 or so people in the audience, mainly campaign guests and students from the University of Denver, will be told to be on their best behaviour.
"The rules are that everyone comes in, takes a deep breath, and holds it for 90 minutes. No noise, no applause, no intervention of any kind," Brown said.
"This is meant to focus on the candidates and their views."
Obama currently leads the national race by six points in the latest Gallup daily tracking poll, and is ahead in most key battlegrounds including Ohio and Virginia.
A Washington Post-ABC News poll out on Monday gave Obama a slimmer 49% to 47% lead, but, tellingly, likely voters in swing states sided with the president by 52% to 41% – Sapa-AFP