She continued: "That's up to Gov. Romney to release his returns" and disprove the rumors.

So far, Romney only has released his 2010 tax return. He has promised to release his 2011 return after it's completed later this year.

At the White House, press secretary Jay Carney deflected repeated questions about Reid's allegations by referring reporters to Reid's office and stressing the "tradition" of presidential candidates releasing multiple years of tax returns.

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Associated Press writer Matthew Daly in Washington contributed to this report.