Updated

Majority Leader Harry Reid, D-Nev., took to the Senate floor Thursday to accuse Mitt Romney of paying no taxes for the last 10 years, amplifying comments he made to media outlets earlier in the week though he's already acknowledged he's "not certain" the claim is even true.

"The word's out that he hasn't paid any taxes for 10 years," Reid said. "Let him prove that he has paid taxes, because he hasn't."

Romney's campaign immediately rejected the statement as "shameful." On Fox News, Romney adviser Eric Fehrnstrom said he doesn't think there's "anything behind it."

"I'm telling you authoritatively, speaking on behalf of the governor, that those charges are untrue. They're baseless. And there's nothing to back them up."

Reid first leveled the charge in an interview with the Huffington Post Tuesday and repeated it on a conference call with local Nevada media Wednesday. Reid says a Bain Capital investor told him Romney didn't pay any taxes for 10 years. Yet he also has not provided the name of the investor and acknowledged to the Huffington Post "I'm not certain" it's true.

The majority leader's comments came in the middle of a scathing critique of the former Massachusetts governor's tax plan. The Tax Policy Center, which Romney has called "an objective third party" in the past, noted that his proposal would give benefits to high-income earners while giving a tax increase to middle-class Americans. Romney's camp has disputed that analysis.

The remarks represent an escalation for Reid, but hew close to earlier remarks he has made on the floor. On July 12, he contrasted Romney's tax disclosure with that of his father George, a Republican candidate in 1968 who released 12 years of tax data. "Mitt Romney can't do that," Reid said, "because he's basically paid no taxes in the prior 12 years."