A leader of the leftist political opposition allied with El Salvador's armed insurgents today flatly rejected any participation by the Revolutionary Democratic Front (FDR) or any of its member parties in national elections scheduled for December. He insisted that dialogue is the only alternative to further prolonged and costly war.

"There is no way," said the long-time political activist, one of eight directors of the FDR, who lives here clandestinely and granted an interview to three reporters on the condition that his name not be published.

"The electoral route in this country is used up," he said. He argued that the government would not respect leftist electoral gains and that campaigning would expose FDR candidates to the fate of five of its leaders murdered in 1980.

The FDR has been allied since 1980 with the Farabundo Marti National Liberation Front, made up of five guerrilla organizations, and its leaders have acted as international spokesmen for the opposition.

The Reagan administration and the Salvadoran government have presented elections last March for a constituent assembly and the newly rescheduled ones at the end of this year for a new president as the only acceptable political solution to the civil war.

The administration has said it would not oppose talks limited to arrangements for the left's participation in the elections, but both Washington and San Salvador have rejected the "unconditional" talks proposed by the left.

Government officials claim that the left does not have the popular support to win a victory at the polls and is therefore trying to seize power through military action combined, if the opportunity arises, with talks to gain a place in the government outside the electoral process.

U.S. Ambassador Deane R. Hinton said in a recent interview that he would be in favor of the left keeping any electoral victory it won, but he did not think it had the chance of a "snowball in hell" of doing that.

The FDR leader called the rescheduled elections a "ploy" and the result of "maneuvers" by Washington with its Salvadoran allies.

"We believe that the elections at this moment, like the others, do not resolve the problems of the war," he said. "We have proposed dialogue as a means of resolving these problems, or we are willing to fight the war to the end with all its consequences. We believe we are going to win the war, and we are demonstrating that."

The Reagan administration, citing serious setbacks to the regime here in the fighting and in the economy, has proposed to increase U.S. military aid to $110 million and send $168 million in economic aid to El Salvador and other Central American nations before Oct. 1. Washington already has spent about $700 million in aid to El Salvador since October 1979.

The FDR leader interviewed here also rejected the possibility of contacts with the government's appointed "peace commission," which was formed by President Alvaro Magana here earlier this month and empowered to talk at least indirectly with the guerrillas.

"That is not representative," he said.

"If leftists were to win the elections, they are not going to give us power," said the leader. "It is a ploy."

"They need to resolve the problem of the power vacuum" that exists in the badly divided Salvadoran administration, assembly and Army, he said. "They need a 'legitimate constitutional government' so they can ask for the direct intervention of the United States."

The man interviewed here today is considered one of the more hard-line members of the group's leadership, but he said he was speaking for the group as a whole in its position toward the elections. An FDR leader with a reputation as a moderate, Popular Christian Party chief Ruben Zamora, also rejected the elections in recent interviews in Washington. "It's just not possible until you go into the process of changing repression in El Salvador. And that's why we need negotiations," Zamora said.

One purpose of the elections, stated privately by State Department officials, is to attempt to woo the moderate elements in the leftist political opposition away from their Marxist-Leninist allies.

This FDR leader said, however, that at present there are "no divisions" within the group over the question of nonparticipation in the December vote.

The FDR was formed as a coalition between the mass-based "popular organizations" developed by the guerrillas and others for nonviolent political and union work in the mid-1970s, center-leftist parties and dissident professionals.

Two of the coalition parties, the Social Democrats and the National Democratic Union, have participated in past electoral campaigns. In November 1980, however, virtually the entire original directorate of the FDR was abducted and murdered here by a death squad working in apparent collusion with government security forces. Since then its current president, Social Democratic leader Guillermo Ungo, has lived in exile. Three other members of the present directorate are political prisoners in El Salvador.

Thus, the issue of "security" remains one of the reasons the FDR leaders give for refusing to participate in the current electoral process. The leader here brushed off talk in the government of a possible amnesty and other efforts to bring the left into the elections, saying that in real terms, "there are no guarantees."

"We are not going to put down our arms," he said.