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.