On Tuesday presidential candidate Mitt Romney fought back against Senate Majority Leader Harry Reid’s (D-Nev.) charge that he did not pay his taxes for a decade, saying the Senate Majority Leader has “lost a lot of credibility” and calling upon Reid to reveal the anonymous source who gave him the false information.

Romney told Fox News that he does not believe Reid has any “credible source.” "I don't know who gave him this line of reasoning, whether it came from the White House or the DNC or a staffer, but he ought to say where it came from, and then we can find out whether that person has any credibility. I know they don't."

Additionally, two major fact-check sites have marked Reid’s claim as unlikely.

PolitiFact wrote that it is “far-fetched” to believe Romney paid no taxes for 10 years, particularly because “Reid has produced no evidence to back up his claim other than attribution to a shadowy anonymous source.”

The Washington Posts’s fact-check consulted tax experts who “say his claim is highly improbable. Reid also has made no effort to explain why his unnamed source would be credible.”

Reid told The Huffington Post in July that a Bain Capitol investor told him Romney did not pay his taxes for a decade - a claim which he has refused to back down from.

Last week Romney told Reid to "put up or shut up" on his unsubstantiated claims. Today the GOP presidential nominee called on Reid to not only shut up, but to reveal his source.

Romney has been criticized for not releasing more of his tax returns. However, Republicans say that Democrats are just using Romney's tax returns to distract from real issues.