UNDER THE GUISE of a "mediation" mission set up by the Organization of American States, the United States is conducting in Nicaragua a bold and unprecedented political intervention pointed at securing President Anastasio Somoza's resignation. The purpose is not always stated so baldly, but President Carter dropped discretion the other day and said the aim was "to set up a government that will have the full support of the Nicaraguan people." A popular government is one without President Somoza, the dictator who is heir to the dynasty put in place by American Marines in the 1930s and who rules by naked force.

Now, many Nicaraguans in opposition know of the history of American support for the Somoza dynasty and of the American fear that replacement of the current Somoza could produce "another Castro." That is president Somoza's high card. The Nicaraguan opposition has broad popular standing and an apparent commitment to a democratic process, and we can understand its suspicious about American policy. But we think the opposition should pay close attention to Mr. Carter's words, to the economic pressure the United States, among others, is applying, and to the potential of the political process that American (and Guatemalan and Dominican) diplomats trying to create. Nicaraguans should not assume the "mediation" mission cannot possibly produce a better result for Nicaragua than the likeliest alternative - further bloodshed, chaos and economic ruin.

To such urgings, the opposition customarily responds that any amount of suffering and loss is acceptable while the Somozas remain: They must go.Here is the hardest part of the mediator's task. They must overcome the impatience and political inexperience of a diverse opposition, while convincing a clever and arrogant President Somoza that his staying means catastrophe for country and family alike. With Mr. Somoza so far refusing to resign and the guerrillas threatening a new offensive, Nicaragua is on the brink.

The Carter administration came to office eager to put intervention, even of a political sort, behind and to let Latins sort out their own affairs. It also had a commitment to democratic ideals - and to the avoidance of undue turbulence. First in the Dominican Republic, where it successfully managed to save an endangered democratic election, and now in Nicaragua, the administration has found it necessary to fudge its commitment to non-intervention in order to pursue its other political goals. The policy is ambiguous, yet, we believe, promising. We hope Nicaraguans give it the chance to work.