"I do not know the party affiliation of the source, how long he invested with Bain, or his relationship to Romney beyond the fact that he was an investor with Bain Capital, as Senator Reid has previously stated," he said. 

Romney has denied any suggestion that he hasn't paid taxes for 10 years. He also has rebuffed Democratic calls for him to disprove that suggestion by releasing more than the two years' worth of returns he's provided. 

Romney, speaking with Fox News earlier this week, said he doesn't believe Reid has a "credible source. " 

Parra said in the radio interview that while Reid is "comfortable" with the source, it's "up to the source" whether to come forward. 

The White House has distanced itself from Reid's comment. Still, the Obama campaign is launching an ad that raises those questions about Romney. 

"Did Romney pay 10 percent in taxes? Five percent? Zero?" the narrator in the ad asks.