Romney, the GOP presidential nominee, said in a Twitter message Biden's remark at a Charlotte, N.C., campaign rally was an admission of President Barack Obama's failures.
"Agree with @JoeBiden, the middle class has been buried the last 4 years, which is why we need a change in November."
Campaign spokesman Curt Cashour called Biden's stray sentence, a day before Obama and Romney hold their first debate, "a stunning admission ... the middle class has been 'buried' under the last four years of this president's policies."
Romney running mate Rep. Paul Ryan told supporters in Burlington, Iowa: "Of course the middle class has been buried. They're being buried by regulations, they're being buried by taxes ... they're being buried by borrowing, they're being buried by the Obama administration's economic failures."
Obama campaign spokeswoman Lis Smith called the attacks "desperate and out of context.''
"As the vice president has been saying all year and again in his remarks today, the middle class was punished by the failed Bush policies that crashed our economy,'' she said Tuesday.
Biden made his remarks when he told about 1,000 people Romney would cut taxes for millionaires and raise them for middle-class families.
He cited an independent study by the Urban Institute and Brookings Institution's Tax Policy Center that found cutting tax rates by 20 percent and closing loopholes to avoid increasing the deficit, as Romney proposes, could be done only by raising taxes on households earning less than $200,000.
The audience booed.
"No, no -- all kidding aside," Biden said. "With all the boos -- I mean, we can stop all that malarkey. Look, guys, this is deadly earnest, man. This is deadly earnest.
"How they can justify raising taxes on the middle class that's been buried the last four years. How in Lord's name can you justify raising their taxes with these tax cuts?"
Romney and Ryan dispute the accuracy of the Tax Policy Center's study and cite studies of their own showing their tax plan will not hurt the middle class.
At a separate campaign rally in Asheville, N.C., later, Biden adjusted his language when he used the word "buried" again.
"The middle class was buried by the [Republican] polices that Romney and Ryan supported," Biden said. "What they're proposing is Bush economic policies on steroids -- that's what this is. But, folks, the middle class is coming back. It's coming back like Americans always do."
More than two in three likely voters say in a United Press International Poll released Monday they expect their financial situation to improve during the next year, with 43 percent of adults saying they prefer Obama while 37 percent say they back Romney.
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