Amid demands for fuller disclosure of his finances, Mitt Romney for the first time confirmed Tuesday that most of his income comes from investments and is taxed at "closer to the 15% rate" — far below the 35% rate on wages for taxpayers in the top tax bracket.

The acknowledgment from the multimillionaire businessman and GOP presidential front-runner came after rival Rick Perry used a South Carolina campaign debate to demand that Romney disclose his tax returns so voters can decide whether "we've got a flawed candidate or not."

It's impossible to pin down specifics about his finances without the tax returns, because the amount Romney deducted from his taxes, the length of time he held investments and other variables are unknown. However, tax law experts who examined Romney's most recent federal financial disclosure report for USA TODAY estimated he earned between $9.8 million and $38.8 million in 2010, with roughly half of the money taxed at a rate lower than that paid by many Americans.