WASHINGTON — Senate Majority Leader Harry Reid repeated his unsubstantiated charge Thursday that Republican presidential candidate Mitt Romney hasn't paid taxes in 10 years, a claim that political watchdogs blasted as irresponsible hearsay. Reid took to the Senate floor Thursday and proclaimed, "The word's out that he (Romney) hasn't paid any taxes for 10 years."

WASHINGTON — Senate Majority Leader Harry Reid repeated his unsubstantiated charge Thursday that Republican presidential candidate Mitt Romney hasn't paid taxes in 10 years, a claim that political watchdogs blasted as irresponsible hearsay. Reid took to the Senate floor Thursday and proclaimed, "The word's out that he (Romney) hasn't paid any taxes for 10 years."

But it was Reid, a Nevada Democrat, who initially put the unproven word out. He told The Huffington Post that an unidentified Bain Capital investor told him that Romney didn't pay taxes for 10 years. "Let him prove that he hasn't paid taxes, because he hasn't," Reid said on the Senate floor Thursday.

Reid, who refuses to release his own tax returns, has refused to provide proof of his Romney tax claim and hasn't revealed the identity of the person who allegedly gave him the information.

Nor did he explain how someone who invested in Bain, which was Romney's firm, would know the details of Romney's personal income taxes.

Some experts on political language and civility said they were aghast that someone in the political leadership of the country — not a bomb-throwing back-bencher — would level such a serious charge without providing any proof. "He shouldn't be making statements like that in public. If you do, produce the evidence or produce the person (who provided the information) in public," said Kathleen Hall Jamieson, director of the University of Pennsylvania's Annenberg Public Policy Center and co-founder of FactCheck.org, a political fact-checking website.

"We need a new category to define inappropriate content. 'Hearsay' is good. What he (Reid) said would not be admissible in court as evidence," she said.

While Jamieson criticized Reid's remarks, she said, "There's a legitimate public interest in Romney releasing his tax returns, because his tax plan could affect people like him."

Romney has released his 2011 tax returns and an estimate of his 2012 returns with a promise to release the final return when it is filed.

Carolyn Lukensmeyer, executive director of the nonpartisan National Institute for Civil Discourse at the University of Arizona, said Reid's remarks are indicative of "a level of insinuating character assassination" that's becoming all too common in today's politics.

"We're actually degrading our institutions," she said. "It's incumbent on the political parties to back up what they said."

Romney's campaign Thursday likened Reid's statements to 1950s anti-communist scare tactics employed by the late Sen. Joseph McCarthy.

"This reminds me of the McCarthy hearings back in the 1950s," Romney adviser Eric Fehmstrom told Fox News on Thursday. "And it was another son from Massachusetts, Joseph Welch, who finally asked the question that should be asked of Harry Reid, which is, 'Have you no sense of decency, sir? Is there nothing that you won't do in the name of dirty politics?' I think it's just shameful."

Reid aides said he has no intentions of amending or walking back his Romney remarks.