NO SINGLE EPISODE in the El Salvador war has bred more bitterness and cynicism, and for better reason, than the nuns' case. Four American churchwomen were brutally slain in December 1980. The former soldiers accused of murdering them have yet to go on trial. Just the other day a judge in San Vicente said more evidence is needed--the latest in a series of delays suggesting that the Salvadoran authorities are playing with the case. Whether the United States has encouraged this attitude by insisting that American aid must flow regardless, or even by giving a few quiet winks of "understanding," is at least a question. The case has become the focus of much heartfelt and political opposition to administration policy, and the judge's latest ruling, coming as Congress debates further aid, is bound to feed the flames.

The Salvadorans argue, essentially, that they are trying and that Americans cannot expect them to ignore the procedural requirements of their law. The argument is not without a certain appeal, but it is foolish of the Salvadorans to continue relying on it. It is no longer possible to maintain that the courts in El Salvador operate in an orbit outside the orbit of official power. The niceness with which the suspects have been treated is in such grating contrast to the savagery loosed upon the nuns, and upon thousands of other victims, that it is impossible to hold the authorities immune from responsibility.

There is a direct link, moreover, to the prospect of a political settlement. The administration's invitation to the left to join government-sponsored elections is premised on a guarantee of the personal security of all candidates and their supporters. How can even the most sorely tempted opposition members be expected to take up this offer as long as the accused slayers of the churchwomen are not brought to justice? This is no mere debating point: it is a hard political reality.

The issue is not merely that the nuns' case embarrasses the administration. It cheapens the United States, the whole nation, to be a party to this farce any longer. Sensitive and sensible people inside the administration grasp this truth quite well. The Salvadoran authorities should be the first to understand the risks to themselves of having a crucial aid decision taken in Washington under the cloud cast by the court in San Vicente.