SENATE MAJORITY LEADER Harry Reid (D-Nev.) ambushed Mitt Romney the other day. In an interview with the Huffington Post, Mr. Reid claimed that he had been told by an unnamed investor at Bain Capital, the firm that Mr. Romney founded, that Mr. Romney had not paid any tax for 10 years.

“He didn’t pay taxes for 10 years!” Mr. Reid exclaimed. “Now, do I know that’s true? Well, I’m not certain.” The uncertainty didn’t stop Mr. Reid from repeating the claim on the Senate floor. Nor did it stop Mr. Reid from rendering a firm judgment. “I mean, you do pretty well if you don’t pay taxes for 10 years when you’re making millions and millions of dollars,” he said in the interview. Later, Mr. Reid said that he had “a number of people tell me that.”

If the senator has any proof, he owes it to Mr. Romney, the presumptive Republican presidential nominee, to put it on the record, now. Otherwise, Mr. Reid ought to pause and reflect on the record of another senator who once claimed to have a list of Communists and spies at the State Department — and could not substantiate it. Mr. Reid’s smear tactics are not unlike those of Joseph McCarthy and deserve equal condemnation. Even in the attenuated and superficial climate of today’s politics, Mr. Reid’s drive-by tactics repel. If he feels so strongly about disclosure, why hasn’t Mr. Reid made public his own tax returns? No need, he says, the Senate financial disclosure is sufficient.

Mr. Romney denies that he paid no tax. His 2010 return, and the estimate for 2011, show that he paid substantial taxes in those years. The Post’s Glenn Kessler points out that Mr. Romney’s investments are not structured to yield zero taxes, although there could have been one or two years with little or no payment. “I have paid taxes every year, and a lot of taxes, so Harry is wrong,” Mr. Romney said.

One way to prove it would be for Mr. Romney, a wealthy and successful businessman, to make public additional years of his tax returns. So far, he has steadfastly refused. There’s no formal requirement to reveal more, but Mr. Romney has deepened voter curiosity about why he won’t — and whether he has something to conceal. As long as he declines, the questions will persist.

So will the demands for disclosure of the identity of Mr. Romney’s well-connected campaign bundlers, to whom he is indebted for vacuuming up truckloads of cash. The candidate knows who they are but is not saying. President Obama is disclosing his bundlers, as did the past two Republican presidential nominees, George W. Bush (twice) and John McCain. On full disclosure, a vital principle of politics and governing, Mr. Romney’s approach is deeply troubling.

Why is it so hard for these two men to grasp that voters are rightly fed up with sleazy rhetoric and sleight-of-hand campaign finance?