El Salvador's military rulers are openly fearful that they eventually may face the same fate as Nicaraguan President Anastasio Somoza, yet they have done little to defuse mounting internal pressures that may ensure that scenario.

Salvadoran politicians and analysts interviewed since Somoza's ouster place highest priority on government guarantees of free and open legislative elections next March as minimum necessities for avoiding a new upsurge of violence there.

Although President Carlos Humberto Romero's government publicly has recognized the need for electoral reforms and active assurances that political freedoms will be respected, it has made little progress either in proposing or implementing the necessary changes.

Government promises were made last May, when nearly 100 persons were killed by security forces during a month of demonstrations that brought wide publicity to the tiny Central American nation.

The disturbances were initiated by leftist groups seeking major reforms, but their cause was picked up by more moderate opposition forces that feared further polarization without greater government flexibility.

In response to both local public outcry and international pressure, Romero called for a "national dialogue" to discuss ways for El Salvador to slowly return to civilian democracy. He spoke of changes in the country's electoral laws, under which conservative military governments have run the country for nearly 50 years, and increased political access for centrist and left-center opposition groups.

Many of El Salvador's most respected leaders, including human rights activist Archbishop Oscar Romero, head of the archdiocese of San Salvador, were not invited to the talks. Others, like the traditional opposition Christian Democrats, refused to attend unless the government made some initial human rights concessions.

In the two months since the talks began, little of substance had emerged and, according to one Christan Democratic leader, as the elections approach "the time for changes is getting short.

"The elections themselves are not the basic problem," he said. "They have to create a climate first in which we can campaign and organize."

Included among the demands are an end to continued repression against church and peasant groups, the read-mission of political exiles into the country and legal reforms.

In one concession the government recently allowed Christian Democratic leader Antonio Morales Erlich to return to El Salvador from longtime exile in Costa Rica. Yesterday, President Romero announced that Jose Napoleon Duarte, the party's 1972 presidential candidate, also could return from Venezuela.

But government critics point out the number of politically motivated deaths and disappearances, allegedly at government hands, has not decreased significantly in recent months.

"They talk about reform and liberty," a peasant union leader said,"at the same time they're still out killing people."

In its most recent bulletin, the San Salvador archdiocese lists 123 persons killed by government security forces ad paramilitaries in June, only 24 less than in violence-filled May and more than twice the number for April.

A Catholic priest killed june 20 by what church officials called "rightwing terrorists" brought the number of priests assassinated during the two-year-old Romero administration to five. On July 14, the offices of El Salvador's only strong opposition newspaper, La Cronica, was firebombed and destroyed by unknown assailants.

Also in June, according to church figures, left-wing extremists are alleged to have killed 14 military and government officials and supporters apparently ensuring the continuing spiral of bloodshed.

The government believes that the threat of continued extremist attacks, particularly in light of what has happened in Nicaragua, is both more immediate and more important than soothing either internal civilian moderates or concerned outside governments.

At the same time, analysts believe, government insistence that mass peasant organizations are both illegal and intimately allied with the guerrilla extremists has prevented it from addressing what oficials themselves admit are very real and volatile complaints concerning economic inequalities and political repression.

Although Romero, an army general, has repeatedly stated his intention to liberalize El Salvador, he is believed to be prevented from doing so by the military establishment that forms his power base. "Romero is a prisoner of the same system that put him in office," one U.S. official said.

According to one high level security officer, the military believes that since the moderate political demands for electoral, personal and union liberties are the same espoused by the guerrillas, "as soon as the guerrillas get those things, they're going to claim victory."

Many of El Salvador's moderate opposition leaders feel that, if change is not forthcoming, the country's guerrillas will not only be able to claim victory, but will have it in fact. CAPTION: Map, no caption, By Dave Cook -- The Washington Post