Mexican President Jose Lopez Portillo's recent call for negotiations to settle the conflict in El Salvador is the latest in a long series of initiatives seeking a political settlement there.

The United States has refused repeatedly to support negotiations between the civilian-military government of El Salvador and the coalition of leftist politicians and Marxist-oriented guerrillas seeking to overthrow it.

Instead of negotiations, the Reagan administration is promoting March 28 elections for the constituent assembly as the most democratic way to achieve peace.

Thomas O. Enders, assistant secretary of state for inter-American affairs, said in a speech in July that El Salvador's leaders, backed by U.S. aid, "will not grant the insurgents through negotiations the share of power the rebels have not been able to win on the battlefield. But they should be--and are--willing to compete with the insurgents at the polls."

The guerrillas and their civilian allies in the Democratic Revolutionary Front maintain that they would be killed if they participated in the elections, citing the murders of many opposition political leaders, including six top leaders of the front.

In a television interview this week, Guillermo Ungo, a leader of the front, said "Right now, there are no conditions for free elections." He proposed negotiations with the Salvadoran government for the formation of a "democratic, broad-based government" that would organize new elections.

The House of Representatives appeared to endorse the concept of negotiations this week with a resolution urging President Reagan to press for "unconditional discussions among the major political factions in El Salvador." But the administration and House Republicans denied that "unconditional discussions" meant negotiations with the guerrillas, and Republicans overwhelmingly voted for the resolution, which passed 396 to 3.

A similar resolution in the Republican-controlled Senate Foreign Relations Committee has been blocked because, according to one member, Chairman Charles Percy (R-Ill.) is under administration pressure to avoid hearings on Salvadoran issues until after the March 28 elections.

Also this week, 104 House members, including 12 Republicans, sent a letter to Reagan urging him to accept the Mexican offer of negotiations.

Some observers of events in Central America, including several members of Congress who have visited El Salvador, say that civilians within the government would like to initiate contacts with civilians in the opposition but are prevented from doing so by the military.

A similar situation prevailed within the opposition at one time, with guerrilla leaders opposing negotiations.

In an interview last week with The Washington Post, Ungo said that as early as July 1980, when leaders of the Democratic Revolutionary Front came to Washington, guerrilla leaders were "talking about military victories, but we the civilian opposition leaders were willing to talk" about a political settlement. The leftist leaders refused an offer by the Salvadoran Catholic Church to mediate because, they charged, the church leadership was progovernment.

At that time, however, the opposition insisted that it would talk only with the United States, not the government of President Jose Napoleon Duarte. Ungo said in an interview in Mexico City in January 1981, "The United States has been demonstrated to be the power behind the Salvadoran throne . . . . We want to talk to the owner of the circus, not the acrobats."

When the opposition leaders visited Washington in the summer of 1980, Enrique Alvarez, who along with five colleagues was killed later that year by Salvadoran soldiers in San Salvador, met with then-assistant secretary of state William Bowdler. During the waning days of the Carter administration, plans were made for opposition leaders to meet with American officials in Honduras, but the meeting never took place.

In January 1981, shortly before President Reagan's inauguration, the rebels launched a major offensive, which fizzled when planned uprisings in Salvadoran towns and cities failed to materialize.

Ferman Cienfuegos, the head of one of the five guerrilla groups making up the Farabundo Marti National Liberation Front, told The New York Times last week that shortly after the failed offensive, the Reagan administration agreed to have top guerrilla leaders make a secret trip to Washington to meet with William J. Casey, director of the Central Intelligence Agency, and then-national security adviser Richard V. Allen. Cienfuegos said the trip was later called off by the administration with no explanation.

A State Department spokesman declined to comment on that report.

The first half of 1981 saw a flurry of activity by political leaders from several Latin American and Western European countries. The Socialist International, made up of social democratic and socialist parties from 59 countries, made a major effort to bring together representatives of the two sides.

In March, the Latin American members of the Socialist International, meeting in Panama, asked West Germany's former chancellor Willy Brandt to urge President Reagan to "promote a political solution" to the war. The Salvadoran opposition coalition announced that it would be willing to negotiate directly with the government. But when Bernt Carlsson, secretary general of the international, met with State Department officials March 3, he apparently received no support for his initiative.

Meanwhile, the Socialist International and the Union of World Christian Democrats tried to arrange a meeting in Europe between Social Democrat Ungo and Duarte, head of El Salvador's Christian Democratic Party. The meeting fell through, reportedly because of opposition from the Salvadoran military and the threat of a right-wing coup against Duarte.

Following the failure of the Western European effort, the presidents of Mexico and Venezuela, two major powers in Central America, met and issued a statement offering their help in reaching a settlement. At the time, the statement appeared to be a hopeful step, since Mexico tacitly had supported the opposition, while the social Christian government of Venezuela had close ties to Duarte.

But the effort died. The two countries apparently failed to agree on how to proceed.

Nicaraguan leader Daniel Ortega presented a proposal from the Salvadoran opposition for a negotiated settlement to the U.N. General Assembly last fall. In December the General Assembly passed a resolution condemning U.S. military assistance to El Salvador and calling for a negotiated political solution.

The opposition made another abortive appeal to the Reagan administration in January when five guerrilla leaders sent a letter to the president renewing an offer to negotiate. The letter was rejected.

The latest major effort was announced Feb. 21 by Mexican President Lopez Portillo at a ceremony in Managua, Nicaragua. The Salvadoran opposition quickly accepted the proposal, but the Duarte government has not yet responded.