An unsubstantiated claim by the Senate's top Democrat that Mitt Romney paid no taxes for a decade continues to dog the presumptive GOP nominee as liberal groups and other Democrats line up behind the assertion.

The Romney campaign is struggling to move past the claim, first made last week by Senate Majority Leader Harry Reid, who cited an unnamed source in asserting that Romney won't release his taxes because he paid no taxes for 10 years.

And now fellow Democrats are jumping into the fray.

Charles Dharapak / AP Republican presidential candidate Mitt Romney pays for a sandwich at Stepto's BBQ Shack in Evansville, Ind.

"Harry Reid made a statement that is true. Somebody told him. It is a fact," House Minority Leader Nancy Pelosi (D-Calif.) told the Huffington Post on Monday in support of Reid.

The Senate majority leader's initial comment last week (also to the Huffington Post) and his subsequent refusal to back off the assertion have prompted reprisals from angry Republicans.

Republican National Committee Chairman Reince Preibus called Reid a “dirty liar” on Fox News this morning. And Romney himself challenged Reid to "put up or shut up" on Friday, when he additionally said he'd paid "a lot" of taxes each and every year.

But short of assailing Reid -- in especially personal terms – Romney and the GOP have found it difficult to move the campaign narrative back to jobs and the economy with headlines about his taxes dominating coverage. Moreover, Romney seems mostly unmoved by the public clamor for his taxes; he has offered no indication that he plans to release records beyond the 2010 records he's already made public, and the 2011 returns he's promised to release.

In the meanwhile, the progressive cavalry has rushed in to Reid’s defense.

"Romney's secretive behavior in hiding his tax returns raises questions about if he actually did pay any taxes or if he had enough corporate loopholes to avoid paying his fair share," said Nevada AFL-CIO executive secretary-treasurer Danny Thompson. "Our members pay their fair share every day - Romney could learn what it really means to support your country from their leadership example. If Romney paid taxes, then let's see it."

"I don't know who Harry Reid's source was, but I do know that this is a question that has swirled around Mitt Romney for this entire campaign," Democratic National Committee Chairwoman Debbie Wasserman Schultz said Sunday on ABC, only stoking the speculative cloud surrounding Romney. "I do know that he could clear it up just like that, lickety-split, by releasing his tax returns."

Romney is nearing his announcement of a running mate, an occasion the GOP candidate is expected to parlay into some sort of campaign swing introducing the VP pick to voters. His campaign is hoping this announcement and the eventual Republican National Convention during the last week of this month will help the campaign move past these questions about taxes.

But reporting this weekend has suggested that Reid, a longtime tactician who’s given to aggravating political adversaries with these types of assertions, takes great relish in his role as attack dog.

It’s one Reid might also cheerfully resume after the vice presidential rollout, as long as Romney’s tax returns remain mostly undisclosed.