WHEN A CASE involving a reporter and a confidential source reaches the courts, the issue is usually the writer's refusal to identify the source he has promised to protect. In a case from Minneapolis that the Supreme Court decided yesterday to hear, however, the facts are unusual. Dan Cohen, who was working as a consultant to the Minnesota Republican gubernatorial ticket in 1982, separately contacted reporters from the St. Paul Pioneer Press and the Minneapolis Star Tribune with information about the arrest record of the Democratic candidate for lieutenant-governor. He was assured of anonymity, but later editors at both papers overruled their reporters and identified Mr. Cohen as the source of the information. He was fired from his job and sued both papers for breach of contract.

The Supreme Court will eventually determine whether the agreement between Mr. Cohen and the reporters was a contract enforceable at law, but the issue is broader than that. It should be noted that The Washington Post Company owns a 26 percent share of the Star Tribune -- though it exercises no control over that newspaper -- but our concern is professional, not financial. Post reporters are authorized to enter into agreements to protect the identity of sources when it is not possible to obtain the information from a named source. In the real world of journalism, we know that far fewer people would provide information to the press, and thus to the public, without the anonymity that protects jobs, personal relationships and reputations. We believe not only that reporters are morally bound to keep their word in such a case but that the free exchange of information would suffer irreparably if, as a general rule, they did not. It is in journalists' own interest not to undermine their reliability with sources.

The court's responsibility, of course, is not to set out theoretical codes of behavior for the press and its sources but to decide a real case with an uncontested set of facts. We regret that this case has arisen and are concerned that in deciding it, understandings that are almost universally accepted about the responsibility of the press to its sources will be altered. The Minnesota Supreme Court decided, correctly we think, that while the newspapers in this case may have had a moral and ethical obligation to protect Mr. Cohen's identity, none of the participants believed he was entering into a legally binding agreement in the commercial or business sense. A moral obligation, they ruled, is not the same thing as an enforceable contract. It is troubling that at least four Supreme Court justices have voted to review this decision.