Senate Democratic leader Harry Reid is unabashed as he makes a serious, unsubstantiated claim about Mitt Romney: that the Republican candidate for president did not pay any taxes for 10 years. That, the Nevada senator says, is why Mr. Romney will not release tax returns beyond those he has already made public, his 2010 return and an estimate for 2011.

Senator Reid, in fact, is so certain he’s doing the right thing that he repeated his charge on the floor of the Senate Thursday night, and put it out in a statement. Reid’s assertion first appeared in a Huffington Post interview published on July 31. He said that about a month ago, he got a call from an investor in Romney’s former company, Bain Capital, who claimed that Romney did not pay taxes for 10 years.

“Now, do I know that that’s true? Well, I’m not certain,” Reid told HuffPo. “But obviously he can’t release those tax returns. How would it look?”

In his Thursday evening statement, Reid repeated his charge, citing “an extremely credible source,” but again did not name the source.

To anyone concerned about the state of political discourse, Reid’s charge is cause for alarm.

“It’s another example of our political leaders not backing up allegations about the opposite party or another candidate with the facts,” says Carolyn Lukensmeyer, executive director of the National Institute for Civil Discourse at the University of Arizona, Tucson. “The American public is really disgusted, and from some polls we’ve done, even ashamed of our national leaders’ inability to solve problems.”

And it’s not that Republicans are angels. “We could easily be having this conversation about some statement from a leader in the other party tearing down [President] Obama,” Ms. Lukensmeyer says.

The likely upshot, she says, is that Americans become further alienated from the political process and just don’t vote.

But the campaigns are not in the business of driving up turnout in the name of civic participation. They just want one more vote than the opposition. If Reid’s tactic is perceived to be successful – Romney’s image remains poor and he loses in November – then chances are voters will be treated to more of same in the future.

Still, political analysts warn that Reid and the Democrats have to be cautious.

“People like a fair game in certain ways,” says Julian Zelizer, a professor of history and public policy at Princeton University. “If there’s a perception that this is about innuendo, rumor, and character assassination, it can backfire.”

He cites Republican attacks on President Clinton in the 1990s, including impeachment, which ended up backfiring on the Republicans.

But Reid is a wily political operator – he almost miraculously won reelection in 2010 despite low job approval – so Romney & Co. can’t dismiss him lightly. In replying to Reid’s charge in an interview Thursday on Fox News, Romney showed rare pique in saying that Reid needs to “put up or shut up” with his allegations.

Some Democrats are giving Reid the benefit of the doubt.

“We know Harry Reid is one tough old boxer,” says Peter Fenn, a Democratic communications strategist. “And you know, he’s not letting up. He clearly thinks he’s onto something here, or he wouldn’t be doing it.”

Watchdogs on political discourse aren’t buying it. Without the evidence, they say, Reid should stop what some are calling McCarthy-esque tactics.

“What he said would not be admissible in court as evidence,” Kathleen Hall Jamieson, director of the University of Pennsylvania’s Annenberg Public Policy Center, told McClatchy Newspapers.

But in the court of politics, anything goes, it seems. Ever since Romney released some of his taxes in January – revealing that he had paid an effective rate of 13.9 percent in 2010, and also that he had accounts in Switzerland and the Cayman Islands – he has been dogged by requests for more. So far, nothing illegal has emerged from Romney’s financial disclosures. And it could be legally possible for him to have paid no tax in some years.

Romney’s spokeswoman, Andrea Saul, has denied that there was any year in which Romney paid zero in taxes. In July, Romney told the National Review he did not want to release more returns because he was “simply not enthusiastic about giving [the Obama campaign] hundreds or thousands of more pages to pick through, distort, and lie about.”

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Romney’s late father, former Michigan Gov. George Romney, began the tradition of presidential candidates releasing tax returns, when he put forth 12 years’ worth in 1968. President Obama has released his tax returns back to 2000. In 2008, Republican nominee John McCain released only two years of returns. But he did not face pressure to release more, likely because he was running as a longtime senator and Vietnam War hero, not on a record in business.

Reid himself has not publicly released his tax returns, but his office says he is not required. He’s not running for president.