If the idea behind inviting President Jose Na poleon Duarte of El Salvador was to bolster administration policy, it's not at all clear he should have come. On "Face the Nation" on Sunday, for instance, he got most of it backwards. Taking a line that the administration itself intermittently realizes is overblown, he portrayed the insurgency as a manifestation of a worldwide communist program without local roots. On the crucial issue of checking the pervasive official violence sponsored by the junta's military and extremists, he gave the impression that it was pretty much beyond him. He could not say how the civilian opposition might be brought into elections conducted by a hostile army that is not under his command and that has for decades spoiled democracy: it annulled his election in 1972.

An extraordinary burden rests on Mr. Duarte, president of a junta whose real power is wielded by a military with close links to a traditional oligarchy. A lawyer and good Christian Democrat, he has undertaken to ride the military tiger that has sat astride El Salvador for at least 50 years, to train that tiger just on a revolutionary left with connections to foreign Communists, and to keep the tiger from consuming him, his fellow democrats and reformers and the general population. The issue long dominating Salvadoran politics is whether the tiger can be ridden. Typically, Mr. Duarte's running mate of 1972 now leads the political opposition. Whether either controls his associated armed forces, however, is questionable. That is what most complicates outside efforts to start a negotiation between them.

Ostensibly, the Reagan administration stands foursquare behind President Duarte. But notwithstanding its professed favor for an electoral solution, the thrust of its policy is toward a military victory over international communism. This puts most of the high cards in the military's hand in its constant internal negotiation with Mr. Duarte. It can turn away when he presses for controls on the official violence, the single factor most retarding junta attempts to widen its popular base.

The administration hopes to build support for its policy by presenting Mr. Duarte as a plucky battler against communism. But why not take a real step to strengthen him against the force he must master first--the military--before the struggle against the insurgency can be effectively prosecuted? Why not, for instance, stop sending American military aid directly to the El Salvador military and channel it straight to President Duarte? The generals can hardly be expected to show any more respect for him than his leading foreign patrons do.