US painting too bleak a picture in Salvador?

Once again the Reagan administration is talking up the leftist guerrilla threat in El Salvador and hinting at possible action to stem it. It is unclear, however, whether the situation in El Salvador has worsened or whether the administration may be preparing a pretext for a new move in Central America.

The guerrillas have made spectacular strikes against the US-supported Salvadoran government in recent weeks.

But there is a gnawing suspicion in Washington that the situation may not be as bleak as some spokesmen paint it. It is suggested that the administration is overestimating guerrilla strength to gain support for eventual moves against leftists not only in El Salvador, but also elsewhere in Central America, the Caribbean, and even Cuba.

Former US Ambassador to El Salvador Robert E. White thinks this is possible. He says any US move against the guerrillas is doomed to fail. But Mr. White, dismissed by Mr. Reagan early in his administration, may have an ax to grind, as one official argues.

That issue aside, however, the administration seems to be floundering in efforts to evolve a coherent US policy on Central America, finding it hard, for example, to speak with a single voice on El Salvador. Among the mixed signals coming from Washington:

* The State Department is officially very upbeat about the military situation in El Salvador. But Secretary of State Alexander M. Haig Jr. is much less so, speaking last week of a ''stalemate'' and arguing that more US military ''help to (El) Salvador is needed.''

* Some US military advisers in El Salvador see improvement in the military situation, but a top US officer in Washington says that the Salvadoran armed forces are losing the impetus to fight that they displayed earlier in the year.

Other administration spokesmen add to this bleak picture, saying the leftists are regaining the initiative lost during their failed ''final offensive'' last January. The bottom line, these officials say, is an underlying concern that things are not going very well there.

Guerrilla forces in eastern El Salvador, for example, appear to be giving the Salvadoran military ''a run for its money,'' as a top Salvadoran officer admitted over the weekend.

They have destroyed tranmission towers and damaged the Puente del Oro, the main bridge over the Lempa River linking eastern El Salvador with the rest of the country. This attack that triggered reports that Cubans were among the guerrillas.

Cuba denies that any Cubans are in El Salvador, challenging Mr. Haig to prove otherwise. Cuban spokesmen add that Cuba has not been arming Salvadoran guerrillas since last January.

But Cuba is deeply worried, its diplomats say, that the US may try some military action against it; reserves have been called up and the country has been put on military alert.

Col. Jose Guillermo Garcia, the Salvadoran defense minister, meanwhile, tends to confirm Cuba's denial of a troop presence in El Salvador and said he had noticed no discernible increase in guerrilla activity.