THERE is, as President Reagan would say, a clear and present danger to U.S. interests posed by the ongoing conflict in El Salvador. But that danger comes not so much from the Salvadoran left as it does from the Administration's refusal to seek a compromise between the guerillas and the right. Washington's plan--announced last week--to increase the number of U.S. military "advisors" in El Salvador is a disturbing sign that the worst may be yet to come.

The desire to send more advisors was underlined by a grim White House briefing of Congressional leaders. United Nations Ambassador Jeane J. Kirk Patrick and National Security Advisor William P. Clark warned Congress that the situation in El Salvador was deteriorating rapidly and the Salvadoran army was waging an increasing uphill battle against the leftists. And Reagan himself conjured up images of the infamous "domino theory" by stating, "We believe that the government of El Salvador is on the front line of a battle that is really aimed at the very heart of the Western Hemisphere, and eventually at us."

The briefing, which was reportedly successful in alarming even some moderate Congressmen, puzzled other Central America watchers who have noticed little change over the past few months. "The Administration has a credibility gap as wide as the Grand Canyon," said Representative Stephen J. Solarz, a Democratic member of the House Foreign Affairs Committee. "Last month they were optimistic, saying the light was practically at the end of the tunnel. This month they're pessimistic, saying the end is at hand unless we take emergency action. So what do we have? A hype."

Even if the White House's assessment of the Salvadoran army's dire straits is correct, sending more advisors and funds will be at best ineffective, at worst catastrophic. Presumably, the Administration has no illusions about routing the leftists; the overwhelming majority of military experts affirm this is an impossible dream. So Washington no doubt hopes only to keep the rightist government in place as long as possible. The advisors and funds are nothing more than temporary measures that demonstrate an acute lack of long term vision.

To maintain the status quo, the Administration will probably have to periodically increase the numbers of U.S. military personnel in El Salvador. The greater American military presence in El Salvador, the greater the likelihood of full-fledged intervention. Such a scenario is frighteningly reminiscent of Vietnam.

The logical alternative for Washington is to press for negotiations between left and right. The guerillas have previously made clear their willingness to talk. But time is running out. The left's confidence increases with each victory, and they may soon reject negotiation. Compromise now is the only way out.