El Salvador's five guerrilla organizations have committed themselves to tougher tactics such as increased kidnapings, along with new efforts to build a mass base of workers and peasants, as part of a plan to merge their forces, rebel leaders say.

This nation's Marxist-led revolutionary movement has suffered since its earliest days from ideological divisions that sometimes have resulted in brutal internecine killings. Now on the defensive militarily and politically, the guerrillas are seeking to put aside past disputes and form a more united front.

The principal split within the Farabundo Marti National Liberation Front, the umbrella guerrilla organization known by its Spanish initials FMLN, is both geographical and ideological. Described simply, it is between the military-oriented guerrillas based in the eastern part of the country and EL SALVADOR'S GUERRILLAS Third in a Series the rebels based here in the north who have shown greater interest in political organizing.

At a meeting of the FMLN's general command in July, there appears to have been a trade-off, according to accounts given by top guerrilla leaders interviewed during a trip behind rebel lines at the end of last month. The "militarists" won support for hard-line tactics such as kidnaping mayors, and they agreed in return to focus more on political work designed to broaden the guerrillas' popular support.

It remains to be seen how much success the FMLN will have in forging "one single army," as the general command said it had agreed to do. Guerrillas of each of the five forces still live in separate camps in the field, or in distinct areas in shared camps such as those on Guazapa Volcano between here and San Salvador.

In addition, the new emphasis on kidnapings and urban commando operations -- such as the slaying of four U.S. marines and eight civilians at a sidewalk restaurant in June -- is likely to strain further the FMLN's alliance with exiled social democratic politicians such as Guillermo Ungo and Ruben Zamora.

While the FMLN is made up of five guerrilla forces, the fundamental division is between the two largest groups. These are the Popular Liberation Forces, based here in Chalatenango province, and the People's Revolutionary Army, based in the northeastern province of Morazan.

The differences between the two are evident in their relations with civilians in their "zones of control," and in the concerns and personalities of their leaders. It has become possible only in the past four months for a U.S. journalist to make that assessment, because the FMLN's leaders did not begin to meet with U.S. reporters until July. They said then, and again in October, that they had made this choice to communicate directly with the U.S. public and thus encourage it to oppose the Reagan administration's strong support for the Salvadoran government.

The Popular Liberation Forces, formed in 1970 by a breakaway faction of the Salvadoran Communist Party, has worked to build a mass base of civilian supporters. Poor peasants living in this area enthusiastically support the guerrillas, and little or no tension is apparent when armed rebels arrive at a village here.

In contrast, relations between the People's Revolutionary Army and civilians living in areas that it dominates in the east appear to be uncomfortable. When six journalists were invited to a news conference in the People's Revolutionary Army's "capital" of Perquin in Morazan province on July 4, the guerrillas organized an obviously staged demonstration by local residents. Some peasants complained as much about the rebels as about the armed forces.

One reason for the difference in relations with civilians was the People's Revolutionary Army's attempt to force peasants to join its ranks last year. The Popular Liberation Forces has avoided forced recruitment, and the other group now admits that it was a mistake. The Popular Liberation Forces' top leader, known as Leonel Gonzalez, was soft-spoken and reserved when he spoke with me here on Oct. 29. He repeatedly emphasized the point that, in his words, "this war is both political and military." He was described as relying on other commanders of his faction, notably Dimas Rodriguez and Salvador Guerra, for military advice.

The People's Revolutionary Army's top leader, Joaquin Villalobos, is widely considered to be the guerrillas' brightest military thinker. He talked almost exclusively about military strategy at the July news conference, as Communist Party leader Jorge Shafik Handal answered questions on political topics. Judging from these two encounters with the FMLN's leadership, Villalobos seemed to have a more forceful personality than Gonzalez.

The guerrilla leaders declined to discuss their fighting strength, but a high-ranking leader of the Popular Liberation Forces who defected earlier this year said that the People's Revolutionary Army has about 2,000 guerrilla fighters and the Popular Liberation Forces has about 1,500. He said that the other forces in the FMLN all are much smaller, with none having more than 500 guerrillas. These three are:

*the Central American Revolutionary Workers' Party, which staged the attack on the marines;

*the Armed Liberation Forces, which is the armed branch of the Salvadoran Communist Party headed by Handal; and

*the Armed Forces of National Resistance, formed by a faction that broke away from the People's Revolutionary Army in 1975.

Since the Perquin "summit meeting," the FMLN's five forces have cooperated more closely in drawing up military and political plans, guerrilla leaders said. The most visible effect was that the Popular Liberation Forces began on Sept. 18 to kidnap mayors of towns in areas either inside or bordering its "zones of control."

The People's Revolutionary Army had started to kidnap mayors in April, but the Popular Liberation Forces had not followed suit, apparently out of a desire to avoid that kind of attack on civilian targets. It was agreed at Perquin that mayors should be kidnaped because "permitting some mayors to carry out their functions was in contradiction with the FMLN's military control in those zones," Gonzalez said here.

There have been other signs of a shift to tougher FMLN tactics since the general command's meeting. In particular, an FMLN commando unit kidnaped President Jose Napoleon Duarte's daughter on Sept. 10, and guerrillas seem to have become more willing than in the past to open fire on passenger buses or trucks during nationwide transportation halts called by the FMLN to disrupt the economy.

The abduction of Duarte's daughter appeared to violate a quiet agreement reached last year between the FMLN and its civilian allies headed by Ungo and Zamora that relatives of prominent persons would be left alone. That accord was reached following the FMLN's abduction of the brother of Defense Minister Gen. Carlos Eugenio Vides Casanova, Gonzalez said.

Gonzalez insisted that Duarte's daughter was seized because of her work with the president's Christian Democratic Party, which the FMLN has declared a target, and not because she is related to the president. Nevertheless, other guerrilla leaders here emphasized that Duarte negotiated with the FMLN only because his daughter's liberty was at stake.

Owing in part to the unity agreements, Gonzalez said, "We've been able to carry out commando operations that have had a great political importance."

Two days after the prisoner exchange, gunmen kidnaped a colonel who is head of the nation's civil aviation authority. Guerrilla leaders here declined to claim responsibility for the abduction, but the Popular Liberation Forces' number two leader, Dimas Rodriguez, said of it, "This is a war."

Gonzalez said the FMLN's transportation blockages had become more effective because the individual guerrilla forces were cooperating more than in the past.

"Now we are doing political work at every roadblock, explaining our positions to the people," he said.

Gonzalez said that most civilians who died during these stoppages were victims of "accidents" that resulted when military personnel in civilian clothes opened fire at guerrilla roadblocks. Five civilians were killed and 18 were wounded during the FMLN's most recent transportation blockage, Oct. 4 to 17.

Perhaps the biggest test of the FMLN's unity plans will be whether the People's Revolutionary Army begins to work to improve its standing with peasants in the east. Without mentioning the People's Revolutionary Army by name, Gonzalez repeatedly emphasized the need to build mass support.

"If in the past we have had an attitude of distancing ourselves from the masses, now we have an opportunity to link up with them," Gonzalez said.

He and other rebel leaders said that all FMLN guerrillas, as well as support personnel such as cooks and nurses, are receiving new political training as a result of the summit.

He also stressed that the FMLN had to build ties with workers in San Salvador, where recent labor agitation has raised the FMLN's hopes that worker discontent will bring them more recruits.