President Jose Napoleon Duarte's apparent voter appeal, his conservative rivals' mismanagement and the armed forces' impartiality combined to reshape Salvadoran politics in the recent legislative and municipal elections, according to observers including Salvadoran and U.S. officials.

The result was a modest turn to the left for the Salvadoran government as Duarte's moderate Christian Democrats extended their power at the conservatives' expense, these sources said. But the armed forces, business community and, most likely, the U.S. Embassy will encourage Duarte to move cautiously in such areas as peace talks with left-wing guerrillas, investigations of human rights abuses and economic reforms.

U.S. policy-makers welcomed El Salvador's elections March 31 as the most peaceful and orderly of the four held since 1982. U.S. sources said they took satisfaction as well from the armed forces' apparent neutrality in the elections.

The military leadership, which generally has been identified with the conservatives in the past, helped Duarte by going on nationwide television after the election to discourage a conservative bid to nullify the vote. But most political sources said the armed forces acted out of what they called a desire for a "serious" electoral process rather than out of partisan support for the Christian Democrats.

While Washington was satisfied with these events, however, there also were indications that the Salvadoran government was edging away from the traditional political right faster than the United States had expected or perhaps desired, according to several Salvadoran and U.S. political observers.

"The fact is that the Christian Democrats' ideology is more reformist and populist than that of the Reagan administration," a left-leaning Salvadoran academic said.

The Christian Democrats soundly defeated the conservatives at the polls. They seized control of the Legislative Assembly, or national congress, and roughly doubled their share of mayoralties and municipal councils, according to widely accepted tallies by the party.

The newly found legislative clout and sharp increase in local patronage will help the president push his party's declared objectives of strengthening the land reform program, carrying out some urban public works projects and continuing his dialogue with the left.

The Christian Democrats will seek to increase the number of people working on farm cooperatives formed when the nation's largest estates were confiscated in the first phase of the 1980 land reform and in the "land-to-the-tiller" program that offers plots to renters and sharecroppers, party leaders said. But they will wait to implement in full phase two of the land reform, under which middle-sized estates would become cooperatives.

The armed forces, on the whole, were expected to continue to resist efforts to prosecute officers who have been implicated in killings of leftist sympathizers. In addition, the military's pledge to enforce the constitution -- which helped to quash the conservatives' electoral challenge -- leads the military also to reject the guerrillas' demands for constitutional changes granting the insurgents a share of power.

As a result, the peace talks are considered unlikely to end the war through a negotiated settlement in the near future. The government's strategy is widely interpreted as using the talks to lure some leftist civilian leaders or combatants into the existing political process and thus to isolate "hard-liners" who would fight on but fade as a threat.

The official vote count was proceeding slowly this week in a large exposition hall at the San Salvador fairgrounds. The Christian Democrats' tallies, accurate in the past, showed their share of the vote rising to an absolute majority of 54 percent from a plurality of 43 percent in the first-round presidential election a year ago.

The Christian Democrats apparently won the bulk of their new strength from the conservative National Conciliation Party, whose share dropped to 8 percent from just under 20 percent last year. Roberto D'Aubuisson's right-wing Nationalist Republican Alliance, or Arena, won just under 30 percent, unchanged from a year ago.

Political leaders hesitated to reach firm conclusions about why the voters shifted to the Christian Democrats and said opinion polls would be necessary to explain the impact of a sharp drop in turnout, perhaps 20 percent.

However, political and diplomatic sources offered several possible explanations, some of them noted before the election:

* The voters' desire to back Duarte, whose personal appeal was reinforced by the Christian Democrats' campaign slogan, "We have the man." The president called his launching of peace talks "decisive" in winning the election.

* Distaste of National Conciliation Party voters for a coalition that their party formed with the more extreme Arena.

* A lackluster campaign by the conservatives, contrasted with an even better effort than last year by the Christian Democrats' capable grass-roots organization.

Arena and the National Conciliation Party were considered to have made an error by proposing to annul the elections. They alleged a handful of seemingly minor irregularities, and the armed forces' 16 top commanders responded by staging the news conference to urge respect for "the sovereign will expressed at the polls."

The National Conciliation Party's leadership split over the challenge, the conservatives' representatives on the Central Elections Council quickly backed off and D'Aubuisson reportedly went on vacation in Guatemala.

The armed forces' action highlighted a rupture, begun with a 1979 coup by reformist officers, between the military leadership and conservative political parties.

"The armed forces have come out to the left of Arena," a U.S. official said.