The United States moved today to put its weight behind the search for a viable governing coalition among El Salvador's three principal political parties, which have seemingly irreconcilable positions on the national issues that Washington has said it considers crucial for an end to Marxist-led insurgency and civil war here.

Preliminary results from yesterday's vote for a constituent assembly showed no clear winner among the three leading parties, the Christian Democrats, the Nationalist Republican Alliance (ARENA) and the National Conciliation Party (PCN). U.S. Ambassador Deane R. Hinton called leaders of the three, along with representatives from three minor parties, to his official residence today for a lunch aimed at beginning to smooth over the differences among them.

The jockeying for position in the 60-seat assembly, to which each party will be assigned a number of seats determined by its percentage of the total vote, clearly has begun. Both President Jose Napoleon Duarte's Christian Democrats and ARENA, led by right-wing former Army major Roberto D'Aubuisson, quickly seized upon a new political buzzword--"national unity"--that belied their longtime mutual enmity.

In the days and weeks ahead, the parties will be "dealing hot and heavy under the table," as one diplomat put it, to shape some sort of workable government. "From here onward," said Christian Democratic leader Julio Adolfo Rey Prendes this morning, "anything is possible."

An alliance between Duarte and D'Aubuisson would have to overcome differences that have long found their parties at each other's throats. ARENA has often insisted that Duarte and the Christian Democrats are Communists, in the same bed as the Marxist-led guerrillas trying to seize power. Throughout his campaign, D'Aubuisson preached an all-out military solution to the conflict, while some Christian Democratic leaders recently have indicated they might be willing to submit to some sort of negotiations with the left.

D'Aubuisson also has not been a fan of the Christian Democrat-supported land reform program whose continuance Washington has said is a key factor for maintaining U.S. military and economic support to whatever government rules in El Salvador.

As for the PCN, it was formed by the Army in 1961 and served as the military power vehicle through two decades of repression and fraudulent votes--including what many believe was the theft of the 1972 presidential election from Duarte. In the political chaos that has prevailed since an internal Army coup in Oct. 1979, it has been unclear exactly where the PCN stands. Many former PCN stalwarts now serve in the ruling Christian Democratic-military junta, while others are believed to have turned toward D'Aubuisson.

Some of the discussions are centering on the idea of putting a "new face" on the Christian Democrats as well as on the parties that might join them.

There is a wide feeling, according to Salvadoran political observers, that Duarte, the current president, has "burned himself" through his participation in the troubled administration of the last two years.

The constituent assembly will have the power to appoint an interim president, and, according to various well-informed politicians, the leading contenders for that post in the current talks are Foreign Minister Fidel Chavez Mena, a Christian Democrat, and Rene Fortin Magana, the businessman leader of the Democratic Action party, currently trailing the three top contenders.

Both Chavez Mena and Fortin Magana are considered moderates with appeal to both conservatives and relatively liberal political elements. Neither is burdened with the record of current or past associations, as are D'Aubuisson and Duarte.

The armed forces, the strength behind whoever ascends to the throne here, will have the power to support or wreck any regime. The United States, which underwrites both the armed forces and the economy, is a major factor.

Hinton insists on the pro forma declaration that the United States does not interfere in this country's internal politics, but the hundreds of millions of dollars that Washington pumps into the fight against the guerrillas give it potentially crucial leverage over any government that emerges from the postelection wheeling and dealing.

Meanwhile the U.S. Embassy is already working hard to smooth the past differences among the parties and, in some cases, even clean up the reputations it had fostered for them.

In an interview earlier this year, Hinton recalled that when D'Aubuisson's party was first formed, the word around the embassy was: "He's just a right-wing extremist; he can't get any support."

This morning Hinton said of D'Aubuisson, "There are people who say he's been dangerous, but he's been a political leader and I think he's behaved very well."

Among the five parties expected to win seats in the assembly when the final vote is tallied, are two commonly predicted coalitions.

One would be between the essentially centrist Christian Democrats, who have a plurality of the votes but have shared responsibility and blame with the military in the Salvadoran government since January 1980, and the moderately conservative Democratic Action party, which has a small but influential constituency in the Salvadoran middle class.

The other possibility--assuming it would be acceptable to the United States--would be between ARENA and the PCN. But either of these coalitions would yield, at best, a bare majority of the 60 seats in the assembly.

One high official in the present government believes that "there will be a stable coalition for some things, but on others they the swing parties will switch from one to another." But the questions facing any government in a civil war require a consensus that such a coalition could rarely guarantee, whether it was opting to go after a full-scale military victory over the rebels or a negotiated way out.

As a result, there is considerable movement now to court the PCN not only by D'Aubuisson, so far with about one quarter of the vote, but also by the Christian Democrats, with 36 percent. The PCN, the old official party, appears to have placed a solid and somewhat unexpectedly strong third in the polling, with 14.5 percent of the ballots counted by this evening.

Preliminary returns show ARENA's absolute dependence upon the PCN to become a major factor in any assembly vote. D'Aubuisson and his friends say they are certain of forming a working coalition, while Christian Democrat Rey Prendes suggested the PCN is more moderate than ARENA likes to think. The leaders of the PCN, meanwhile, were keeping their own counsel.