The Salvadoran rebel movement has begun discussions on the possibility of participating in elections organized by the U.S.-backed government of President Jose Napoleon Duarte, according to insurgent leaders.

The proposal, which would mark a major concession in rebel demands, has been advanced by leaders of the movement's political wing, the Democratic Revolutionary Front, but has not been accepted by the more adamantly leftist military wing, the Farabundo Marti National Liberation Front, these leaders say.

Chances for acceptance by the entire rebel alliance depend chiefly on the course of a dialogue with Duarte's government opened a month ago at a precedent-shattering meeting in the Salvadoran hill town of La Palma, according to Ruben Zamora of the Democratic Revolutionary Front.

While debating the issue of participating in elections, the rebel movement has decided to break its negotiating position into phases, concentrating first on proposals for small steps to "humanize the war," Zamora said.

This approach, likely to dominate the coming round of talks, opens the way for concrete improvements in the lives of Salvadoran civilians, he said, while at the same time allowing the rebel movement to postpone a divisive debate on what to do about elections and other issues in the future, should the talks move ahead that far.

The second rebel-government meeting is scheduled to take place within the next two weeks. Rebel officials said they have proposed it be held Nov. 27 in San Salvador but so far have not received an answer from Duarte. Indirectly, they have word that Duarte wants the gathering to take place earlier, possibly by the end of next week, but the site is still undecided, they added.

To improve the atmosphere for the talks, the rebel movement is considering announcement of a unilateral truce for the Christmas season, Zamora said in an interview here. The gesture is one of several concrete issues the insurgents plan to discuss with Duarte's government in a first phase of a dialogue they hope will evolve into negotiations on a political solution to the five-year-old civil war, he added.

"Our principle is that the dialogue should eventually be converted into negotiations," said Jorge Villacorta, the Democratic Revolutionary Front representative in San Jose, Costa Rica.

In a televised debate with guerrilla representatives in California Wednesday night, Christian Democratic Party leader Julio Adolfo Rey Prendes appeared to rule out a cease-fire, saying the government had called only for a "cessation of violence," not a formal truce.

The talk of participating in elections without a change of government first would be an important change in the rebels' position. Previously, the unanimous rebel position had been that only a new government, including insurgents and "broad participation" by Salvadoran groups, could organize new elections.

Duarte and the Reagan administration repeatedly have denounced this demand for power sharing, saying the rebels were "trying to shoot their way into power." Even the Democratic Revolutionary Front's suggestions, however, contain demands for substantial changes in what would become in effect a provisional government with Duarte at its head to prepare elections that would put his office up to another vote.

The elections could not be held in March as now scheduled, for example, because insurgent opposition leaders would need more time to organize politically, rebel officials said.

"If the FDR-FMLN is to participate in elections in El Salvador," Zamora said, using Spanish initials for the movement's political and military wings, "we have to transform what is an underground organization into an open organization, and you can't do that in a few months."

In addition, rebel leaders would have to participate in an electoral council and help draw up new electoral laws to remove what the officials said were opportunities for abuse by the Salvadoran Army, they declared.

Most important, they added, security arrangements would be necessary to guarantee the safety of rebel leaders who returned to live and organize politically in El Salvador. This issue would be particularly difficult for military commanders involved in a bloody conflict against the Army for nearly five years, they declared, and it forms the major objection of guerrilla leaders who doubt that such arrangements can by worked out with Salvadoran Army commanders.

Aside from the Christmas truce, the rebel movement has decided to suggest in the next round of talks that the first phase of dialogue cover tighter observance of the Geneva accords on warfare by guerrillas and Army alike and better government controls over death squads that, according to the rebels, are run from within government security agencies, Zamora and Villacorta said.

If accords could be reached on these issues, the talks could move on to subsequent phases including such other "humanizing" steps as a halt to sabotage of civilian traffic on Salvadoran roads, they added. Both leaders emphasized, however, that these ideas are only goals in a dialogue that has just made a tentative beginning and remains fragile.

Duarte's opening proposal that guerrillas lay down their arms under an amnesty program and take part in legislative elections next March as a political party, for example, has not been discussed seriously because it amounts to surrender, they said.

The mechanics also remain difficult. Zamora said the insurgent leadership has become worried over lack of response to its proposals for the second meeting, which he said were conveyed to Duarte through the Salvadoran Catholic Church two weeks ago.

In them, the rebels suggested the "mixed commission" agreed on at La Palma should hold the next gathering with second-level officials to prepare another high-level meeting in mid-December including Duarte and the top guerrilla leadership.

If Duarte insists on top-level representation at the next meeting, Zamora explained, this could raise problems for guerrilla field commanders hard pressed to move quickly to a meeting site from rebel-held areas. The paramount guerrilla commander, Joaquin Villalobos, declared that this had prevented him from attending the La Palma meeting Oct. 15.