SAN SALVADOR, NOV. 20 -- Despite death threats, top political allies of leftist rebels are preparing to work openly in El Salvador after seven years in exile, a move that could mean a rupture in their alliance with the armed Marxist-led insugency.

The scheduled return of leaders of the Democratic Revolutionary Front (FDR) is viewed by many diplomats and politicians as the first important change in Salvadoran politics since President Jose Napoleon Duarte was elected in 1984.

It is one of the few tangible results in El Salvador of the regional peace plan, signed Aug. 7 in Guatemala City by five Central American presidents.

"We have decided that for us, elections are the only mechanisms for taking power," said Hector Silva, a representative of the leftist coalition who returned to El Salvador in 1985 to begin laying the groundwork for the others' return.

Ruben Zamora, vice president of the front, is scheduled to return Saturday and Guillermo Ungo, the group's president, on Monday.

Because of the thousands of political murders in El Salvador in the past eight years, mostly by right-wing death squads, there is great concern over the returning exiles' safety, and both will be accompanied by international delegations. The coalition's current leaders have lived abroad since 1980 when its top leaders were assassinated here.

According to diplomats and analysts, the move is a severe blow to their allies of seven years, the Farabundo Marti National Liberation Front (FMLN), a coalition of five Marxist-led armies seeking to overthrow the U.S.-backed government.

Diplomats who monitor the insurgents said there is deep resentment against those returning because it will deprive the guerrilla coalition of some of its top fund raisers and make it more difficult to justify the armed struggle to European and Latin American supporters.

"This is a divorce, they just have to keep up appearances for a time," said one knowledgeable Western European diplomat.

While Democratic Revolutionary Front leaders publicly denied a split, they have taken stands in recent days that put them sharply at odds with the guerrillas on fundamental issues.

{The Salvadoran armed forces resumed operations against the guerrillas Friday, ending a unilateral cease-fire declared after the Central American peace plan was signed, United Press International reported.}

On March 20, 1988, there will be elections for municipal offices and all 60 seats of the unicameral legislature.

Silva said he had received many anonymous telephone death threats in recent weeks, as word of the exiles' return spread, but declined to speculate on who was responsible.

The threats gained credibility after the Oct. 26 assassination of Herbert Anaya, president of the unofficial Human Rights Commission.

Archbishop Arturo Rivera y Damas blamed Anaya's murder on death squads, and urged FDR leaders to delay their return because of the danger.

The exiles know they are taking a risk by giving Duarte a political boon when his popularity has slipped to one of its lowest points.

Duarte, in his most recent statement several weeks ago, said those returning would have to renounce all ties to the guerrilla front and publicly accept the recently approved amnesty, something the FDR leaders refuse to do.

But Julio Rey Prendes, minister of communications and culture and a close Duarte adviser, said in an interview yesterday that they were automatically covered by the amnesty.

"They are forgiven whether they want to be or not," he said. "They will have no problems from the government in returning."

In a separate development, a military judge ruled admissible an appeal filed by the attorney general's office and the U.S. Embassy to deny amnesty to three suspects in the killing of four U.S. marines in 1985. Judge Jorge Alberto Serrano said the case would now go to a military court for a final ruling on whether the three qualify for amnesty. The three remain in custody.