Mitt Romney lashed back at Harry Reid on Thursday in an interview with Fox News' Sean Hannity, saying the Senate majority leader needs to "put up or shut up" after airing allegations about Romney's taxes.

Reid, a Nevada Democrat, first raised eyebrows Tuesday by saying in a news interview that someone had told him Romney went 10 years without paying taxes. He would only identify his source as an investor in Romney's former venture capital firm, Bain Capital, and he acknowledged, "I'm not certain" it's true.

That didn't stop Reid from taking to the Senate floor Thursday to accuse the Republican presidential candidate again of paying no taxes, part of a broader Democratic attack on Romney for declining to release more than two years of tax documents.

"The word's out that he hasn't paid any taxes for 10 years," Reid said. "Let him prove that he has paid taxes, because he hasn't."

But Romney forcefully denied Reid's allegations on Hannity's radio show Thursday.