El Salvador -- one of Ronald Reagan's first foreign-policy challenges

El Salvador is likely to be Ronald Reagan's first Latin American crisis. But events there are changing so rapidly and dramatically that any plans Mr. Reagan and his advisers draft now may well have to be sharply redrawn by the time the new President takes office Jan. 20.

The killing this past week of six top leftist leaders -- including the flamboyant Juan Chacon -- has significantly altered assessments of the current situation in the embattled Central American country.

The left obviously has suffered a new major loss. Already reeling from months of steady defeats at the hands of both the joint military-civilian junta and rightist paramilitary forces, and shaken by serious internal divisions, the left could ill afford to lost its top leadership.

Those killed were members of the leftist Democratic Revolutionary Front, a coalition of 18 leftist organizations. They were trying last week to reach common ground on which to rekindle efforts against both the junta and brutal rightist forces.

The leftist leaders were meeting in a Jesuit-run high school three blocks from the US Embassy in San Salvador when more then 200 heavily armed supposed rightists abducted them and 20 others. The next day, the bodies of the six leaders were discovered.

The development raises questions not only for El Salvador, but also for Central America as a whole and for the United States: What effect will the Salvadorean left's declining fortunes have in neighboring Guatemala, where a similar struggle is developing? What effect will it have on Nicaragua, where a leftist government struggles to stay afloat?

For the US, the left-right confrontation in El Salvador and the prospects of the joint military-civilian junta are a major concern. The outgoing Carter administration, while critical of human-rights violations by successive Salvadorean governments, has come out strongly in support of the present junta.

In recent months, Washington stepped up economic and military aid to the junta -- and although the military aid has not included much weaponry so far, the aid has helped infuse a new sense of purpose into military efforts to deal with leftist guerrilla activities.

Advisers to President-elect Reagan want the US to dend combat equipment to the Salvadorean military -- and this is likely to begin soon after the new administration's inauguration.

At the same time, those close to Mr. Reagan are being careful to indicate that they would not want to see a takeover by the ultra-right in El Salvador. For now, these Reagan associates go along with the Carter approach of propping up the military-civilian junta as the best solution to the imbroglio in El Salvador.

It remains to be seen whether the Reagan people, once in office, will continue this support for the junta, which is under attack from both the left and the right.

There is some speculation in Central America that rightists throughout the region are planning major attacks on the left before Mr. Reagan becomes president. That would, in a way, present the new President with a fait accompli. Some observers see the killing of the Salvadorean leftist leaders as part of this effort, which is also apparently aimed at discrediting the moderate military-civilian junta.

Many of the country's rightists are bitter over the junta's modest economic and social reforms, which include nationalization of the banking and commodity industries as well as the breaking up of large estates and the distribution of the land to peasants.

Over the past several weeks, some Salvadorean rightists have met with Reagan advisers, making known their opposition to the junta and some of its reformist efforts. What impact such contacts will have on the new administration remains to be seen.

For the moment, however, the key Salvadorean development is the serious losses the left continues to sustain. Not only have leftists lost their top leaders, but also many top battlefield commanders have been killed in recent months. The lest has been defeated in numerous battles with rightists and government forces. The left also has lost large caches of arms and ammunition. Moreover, the junta's reform programs have to a degree pulled the rug out from under the left, which has long advocated such programs.

Perhaps most important, however, the average Salvadorean is growing tired of the five years of incessant conflict that have left at least 25,000 dead, some 8 ,000 to 12,000 this year alone. Leftist efforts to bring out the common folk in mass protest demonstrations, ever so successful in 1978 and 1979, have simply fizzled this year.

El Salvador's left is clearly on the run. It makes little difference who was ultimately responsible for the deaths of the six leftist leaders Nov. 27; the incident was yet another evidence of just how desperate the leftist plight has become.