Harry Reid has provoked outrage among liberals as well as conservatives, who seem to believe he has violated propriety by repeating gossip about Mitt Romney’s taxes. The Senate leader says someone connected with Romney told him that the Republican candidate paid no income taxes for a period of 10 years. Offended by Reid’s audacity, commentators on the right have indicted him for “McCarthyism,” while others on the left have accused him of inventing the whole story.

Evidently the chief complaint against Reid — aside from aggressiveness unbecoming a Democrat — is that he cited “an extremely credible source” who he has so far declined to name. Some journalists have gone so far as to suggest that Reid must be lying because he won’t identify the source.

Despite all this righteous tut-tutting among the great and the good, in newspapers and magazines as well as on television, Reid’s critics simply have no way of knowing whether he is telling the truth or not. From the beginning, Reid himself admitted forthrightly that he has no way of being absolutely certain whether what he was told is factual or not, although he believes the person who said it was being truthful.

Many of Reid’s critics work for news outlets that rely on unnamed sources every day, of course, publishing assertions that range from the mundane to the outlandish. It is hard to see why an unnamed source quoted by a daily newspaper or a monthly magazine — or hidden behind a screen in a TV studio — is more credible than a person whispering in the ear of a United States Senator.

Indeed, several of the news outlets now barking at Reid have suffered their own episodes of scandalous embarrassment owing to the exposure of invented sources and quotes (see Jayson Blair, Stephen Glass, etc. etc. etc.). Yet they nevertheless continue to publish quotes from such unnamed individuals. After all, where else would Reid have learned that this is acceptable conduct?

Meanwhile, Romney’s response is to demand that Reid “put up or shut up” — that is, reveal the name of his source. But that would prove nothing. As Reid has pointed out, only the former Bain executive can demonstrate conclusively that suspicions about his tax history are unfounded. Although the irritated Romney retorts that he has “paid a lot of taxes,” his denial won’t suffice as proof either. He could have paid hefty real estate taxes on his various homes and sales taxes on his purchases of cars, car elevators, powerboats, and other luxury goods, among other levies, while paying little or no federal income tax.

Obviously, it would be simple for Romney to disprove Reid’s statement, which is unlike McCarthyite accusations that involve someone’s personal associations or state of mind. The necessary evidence is not only within Romney’s possession but is also material that candidates in his position normally release to the public and that the public expects to see. It is material that he previously surrendered to Senator John McCain’s campaign staff in 2008, when they were vetting him for a possible vice presidential nomination. (For now, they are conspicuously silent on the Reid controversy.)

There is a legal doctrine that applies to Romney’s current behavior, as Indiana attorney John Sullivan points out — and it doesn’t place the burden of proof on Reid:

At law, if a person in control of evidence refuses to produce the evidence, then the jury is instructed that there is a presumption that the evidence would be against the party failing to produce. It is called the “Missing Evidence” instruction.

The missing evidence is in Romney’s grasp, yet he insists that he will never produce it. Does anyone need instruction from a judge to make the correct inference?

Joe Conason is the editor in chief of NationalMemo.com.