On Thursday, Biden slammed Romney at a campaign stop in New Hampshire for promoting a "Romney Rule" on taxes that would benefit only millionaires like him - a harsh comparison to the "Buffett Rule" Democrats are promoting.

"The Buffett Rule says that multi-millionaires should pay at least the same percentage of their income in taxes as middle-class families do," Biden said Thursday. "The Romney Rule says the very wealthy should keep the tax cuts and loopholes they have, and get an additional, new tax cut every year that is worth more than what the average middle class family makes in an entire year."

Anticipating the pressure to release Romney's tax returns, the former Massachusetts governor's campaign said Obama's campaign was diverting attention from a failed economic record.

"It's no surprise with the worst job creation record in modern history that President Obama would try to distract Americans from the real issues with a series of sideshows," Romney campaign spokeswoman Andrea Saul said. "Gov. Romney has already released his 2010 return and an estimate of his 2011 income and taxes. He will release his full 2011 return when it is filed."

Obama's 2010 tax return showed the president earned $1,728,096 and paid $453,770 in income taxes.