Rebellion in El Salvador; Guerrillas: tough warriors, fragile unity

They began in disunity. And for a long time, El Salvador's leftist guerrillas fought separately - sometimes even among themselves.

But for the past 18 months a loose unity, based on their common goal of bringing down the Salvadoran government, has held the leftists together. The top guerrilla commanders are Marxists, some of whom have been trained in Cuba, and this also unites them.

As the Salvadoran civil war escalates, however, questions about the guerrilla movement's fragile unity are surfacing.

Equally important is the question of who ultimately controls the movement. This question is all the more urgent after Secretary of State Alexander M. Haig Jr.'s claim this week that the United States has ''overwhelming and irrefutable'' evidence that the insurgents are controlled from outside the country by non-Salvadorans - although Secretary Haig refused to specify who these non-Salvadorans are.

Although cracks in the guerrillas' facade of unity have been evident all along, there now are reports of feuding among their leaders over tactics and perhaps over status within the movement. This takes place as more radical elements with connections with Cuba, Nicaragua, and the Soviet Union appear to be gaining the upper hand - a situation that could fit into Secretary Haig's assertion.

The guerrillas' original move toward unity resulted, as they admit, from awareness that hope for eventual victory over the Salvadoran Army depended in part upon remaining united.

The example of the Sandinistas in Nicaragua is very much in the minds of the Salvadorans. Not until the three disparate Sandinista factions united did they begin to score any significant successes against the dictatorship of Anastasio Somoza Debayle.

But El Salvador is not Nicaragua, nor are the Salvadoran guerrillas formed out of the same sort of idealistic mold that helped unify Nicaragua's Sandinistas. They have similar goals - the setting up of a pluralistic society and a mixed socialist-capitalist system - but the Salvadoran guerrillas, perhaps because they do not have the business community with them as did the Sandinistas , talk a much more determined socialist line than the Sandinistas.

Moreover, the Salvadorans have much less public support than the Sandinistas received in the final months of their struggle against General Somoza. This is widely seen as one of the reasons they have repeatedly opposed the government's plans to hold an election to name a constituent assembly on March 28 - and why they have actively sought to undermine election plans.

The guerrillas, of course, are not without some public support. They have proven strong and tough, as the Salvadoran Army attests. They have about 6,000 men and women under arms. There are another 15,000 support people who run safe houses, serve as couriers, act as paramedics, and do the myriad chores that keep an army moving.

The guerrillas have proven effective on the battlefield, regularly holding the Salvadoran Army at bay in parts of Chaletenango and Morazon provinces. They are doing so at this moment on the rugged ridges of 3,000-foot Guazapa Volcano, a scant 20 miles from San Salvador, the country's capital city.

There are five recognizable groups represented by the unified guerrilla command, the Frente de Liberacion Nacional Farabundo Marti, named after a guerrilla leader of the 1930s. They are:

* The Fuerzas Populares de Liberacion (FPL), led by Salvador Cayetano Carpio, a former baker who goes by the name ''Marcial.'' Long linked with Havana, he founded the FPL in 1970, manning it mainly with youths from rural areas. The FPL is most active in the region west of the Lempa River, which includes the rugged terrain of Chaletenango Province. The Salvadoran Army has not been able to wrest control of the province from the guerrillas, although government troops are stronger there today than in the past.

* The Ejercito Revolucionario del Pueblo (ERP), led by Joaquin Villalobos, a former university student whose organization spun off from the FPL five years ago. The ERP concentrates in the area east of the Lempa River and is particularly strong in eastern Morazon Province. Many of its members are young men from urban areas.

* The Fuerzas Armadas de Resistencia Nacional (FARN), headed by Fermin Cienfuegos. This group is an offshoot of an earlier attempt at leftist unity, the defunct Frente de Accion Popular Unitaria, set up in 1974. Guerrillas interrogated near Santa Ana in western El Salvador last year claimed to be members of the organization.

* The Partido Comunista de El Salvador, led by its secretary-general, Jorge Schafik Handal. This group dates from Stalinist times and has long been allied with Moscow. More a political than a guerrilla organization, the party also has some young cadres who are thought to have been involved in the fighting in the past year.

* The Partido Revolucionario de Trabajadores Centroamericanos, headed by Roberto Roca (his name is a pseudonym). This group, a small Trotskyite offshoot from the Communist Party, is an effective propaganda organization that includes the clandestine Radio Liberacion.

The guerrillas also have a political arm - the Frente Democratico Revolucionario (FDR) composed of both Marxists and non-Marxists opposed to the military-civilian junta that rules El Salvador today.

Based in Mexico City, the FDR is a shadow government that eventually hopes to take power. Serving as a propaganda organ for opponents of the junta, the FDR is headed by Guillermo Manuel Ungo, a former member of the junta.