Mitt Romney called on Harry Reid Tuesday to reveal the source for his claim that the Republican presidential candidate has not paid taxes for 10 years, stepping up pressure as two major fact-check sites ruled Reid had no basis for the "incendiary" allegation.   

Romney, in an interview with Fox News, said he doesn't believe Reid has a credible source but urged the Senate Democratic leader to reveal who it is. 

"I don't really believe that he's got any kind of a credible source," Romney said. "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." 

He said Reid also has "lost a lot of credibility." 

Romney's latest comments came as two major fact-check sites knocked down Reid's comments, which the senator amplified by repeating them last week on the Senate floor. Reid has said his source is an investor with Romney's former venture capital company Bain.