Hours after Mitt Romney said he's never paid less than 13 percent in taxes over the past decade, President Barack Obama's re-election campaign bluntly told the Republican candidate to "prove it" by releasing his returns.

"We would say: 'Prove it, Gov. Romney," Obama campaign spokesman Ben LaBolt told reporters on a conference call.

Romney told reporters at an impromptu press conference in South Carolina that he's never paid less than a 13 percent tax rate over the last 10 years and blasted Democratic critics, including Senate Majority Leader Harry Reid, for leveling the "totally false" charge that he's paid no taxes.

"He has the ability to prove his claim," LaBolt said.

"Look, this is a candidate who has defied bipartisan precedent: His own father put out 12 years' worth of returns, Gov. [George W.] Bush put out returns for every year of when he was governor," the spokesman added.

"Yet Gov. Romney has only put out two years' worth of returns. And the fact is the American people deserve the opportunity to look through those documents and to make their own conclusions," LaBolt said.

The Obama campaign has hammered away at Romney on the tax issue as part of an effort to use his personal finances as a weapon in the campaign. The president, his re-election prospects weighed down by the sour economy, has made the argument that he will fight for the middle class while Romney will champion the very rich.