"I don't really believe that he's got any kind of a credible source," Romney told Fox News. "I don't know who gave him this line of reasoning," adding that Reid, the Senate Democratic leader, has "lost a lot of credibility."
Reid says his source is an investor in Romney's former venture capital firm Bain Capital.
Romney's comments came as The Washington Post and the fact-checking site PolitiFact denounced Reid's claims, PolitiFact referring to them Tuesday as "an extreme claim with nothing solid to back it up."
"Tax experts say his claim is highly improbable. Reid also has made no effort to explain why his unnamed source would be credible, so in the absence of more information, it appears he has no basis to make his incendiary claim," the Post said Tuesday.