Washington — Jose Napoleon Duarte's apparent victory in Sunday's election in El Salvador is expected to boost substantially the Reagan administration's chances of securing additional military aid for El Salvador.
Mr. Duarte, a Christian Democrat, is widely regarded in the Congress as a moderate, much to be preferred over his opponent in the runoff election, Roberto d'Aubuisson. Duarte has proposed a ''dialogue'' with representatives of the guerrilla left aimed at bringing the left into elections and ending the war.
But there is also an awareness here that Duarte's power and his room to maneuver may be sharply restricted by the Salvadorean military and the right-wing groups that oppose him. The Reagan administration is expected to make the argument that additional American aid at this juncture is likely to bolster Duarte's position and make it possible for him to bring right-wing abuses under control. Whether Duarte can move El Salvador any closer to a peaceful resolution of the war remains very much in doubt, however.
Most of the signs elsewhere in Central America point toward continued and possibly intensified armed conflict:
* State Department officials report an increase in arms going from East-bloc nations to Nicaragua as well as an increase in ammunition flowing into El Salvador. The Salvadorean guerrillas will be in a position to launch major offensive actions in the fall, the officials say.
* Tension has risen along the Costa Rican border following a flare-up there. The United States has promised that its aid to Costa Rica will move on an emergency basis. State Department officials say that Nicaragua apparently hoped to intimidate Costa Rica by attacking a border town. Nicaragua denied attacking Costa Rica but expressed concern over rebels taking shelter in Costa Rica who are reported to be getting US Central Intelligence Agency backing.
* Within Nicaragua itself, opposition politicians report little hope that the elections planned for November in that leftist-led nation will lead to a process of reconciliation between ruling Sandinistas and their opponents.
* The so-called Contadora peace process, which has been the great hope of many US congressmen, is moving forward at an agonizingly slow pace. Foreign ministers from El Salvador, Honduras, and Costa Rica recently placed much of the blame for a lack of substantive progress on Nicaragua. The Nicaraguans have accused the United States of working through its Central American allies to impede progress.
The Contadora group, which consists of Mexico, Colombia, Venezuela, and Panama, recently ended an inconclusive round of talks in Panama City. Ministers said they planned to move forward to incorporate more specifics in their peace proposals.
In Nicaragua, the Sandinistas have accused Roman Catholic bishops in Nicaragua of establishing a political position that supports the ''aggressive plans of the American administration.''
In a pastoral letter on April 22, the bishops called for a dialogue between the Sandinistas and their armed opposition, an opposition that includes a CIA-backed force of more than 10,000 men fighting in the north of Nicaragua. If there is no such dialogue, said the bishops, ''there will be no possibility of a settlement.'' The Catholic radio network in Nicaragua was not permitted to broadcast the pastoral letter, according to officials here.
Several Christian Democratic politicians from Nicaragua who visited Washington last week said they had little hope that the Sandinistas would permit fair elections in November. The opposition politicians said that despite an announced ''opening,'' the Sandinistas have maintained a state of emergency and strict press control. They said they were not permitted to organize outdoor meetings and were subject to harassment from government supporters. ''It is hard to imagine free elections,'' said Azucena Ferrey Echaverry, vice-president for international affairs of the Social Christian Party.
The Reagan administration, meanwhile, apparently plans to take immediate advantage of the Salvadorean elections to renew its push in the Congress for aid to El Salvador. President Reagan has tentatively scheduled a major address for Wednesday evening. And officials are discussing plans to invite Duarte, should his election be confirmed, to come to Washington later this month to help convince the Congress that more aid is needed.