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Mitt Romney has a message for Harry Reid: "put up or shut up."

The Senate Majority Leader twice this week has repeated a wildly speculative rumor, that Mitt Romney did not pay any taxes for 10 years.

"The word's out that he hasn't paid any taxes for ten years," Sen. Reid, D-Nev., said of Romney on the Senate floor Thursday. "Let him prove that he has paid taxes, because he hasn't."

Reid was repeating what he had originally said in an interview with the Huffington Post earlier in the week, in which he recalled that a month ago someone who invested with Bain Capital called his office to say that Romney "didn't pay any taxes for 10 years."

Romney is the former CEO of Bain Capital, but left the company in 1999. Reid freely admitted he couldn't substantiate the charge and he didn't really even know if it was true. But he repeated it anyway.

"He didn't pay taxes for 10 years! Now, do I know that that's true? Well, I'm not certain," said Reid to the Huffington Post based on information provided, he says, by an informant. "But obviously he can't release those tax returns. How would it look?"

Today Romney fired back at Reid working the rumor mill and called on the Majority Leader to reveal his informant.

"I have paid taxes every year and a lot of taxes, a lot of taxes," Romney said today in Las Vegas. "Harry is simply wrong, and that's why I'm so anxious for him to give us the names of the people who have put this forward. I wouldn't be at all surprised to hear the names are people from the White House or the Obama campaign or who knows where they're coming from."

Or better yet put simply, "Harry Reid really has to put up or shut up," Romney said.

Democrats have criticized Romney for not releasing more than two years of his tax returns. But there has never been anything to suggest that Romney's tax bill was zero. It is possible, however unlikely, that someone like Romney who pays most of his taxes in the form of capital gains could legally pay no taxes in the year after a big loss.

The presumptive GOP candidate has said he always paid all of the taxes he was required by law to pay. But he has said releasing more tax returns would likely not quiet critics.

Romney has so far released his 2010 returns and an estimate for 2011. Those two years are all Romney says he will release, arguing that no matter how many years he releases critics will always want more.