El Salvador's Christian Democrats are likely to move after Sunday's election to form a "government of national unity" that could negotiate with leftist insurgents for an end to the civil war here, according to senior members of the party.

These officials, whose party was brought into the ruling junta by the military two years ago, said privately that the exact manner in which this might be carried out is still under discussion. At the same time, the question of whether the Christian Democrats will win enough seats in the 60-member constituent assembly to carry out any such program remains uncertain.

Great pains have been taken to make sure not only that the voting is fair, but also that it is recognized as such. Hundreds of journalists have come here along with official observers from 20 countries and unofficial ones from about 60.

Some of the eight members of the official U.S. delegation were already holding crowded press conferences today, before the first ballot is cast.

"I'm convinced that this election is going to be a lot more honest than a lot of elections," said Notre Dame University President Theodore Hesburgh.

But in a pattern that has become more pronounced each day as the elections draw closer, the bustle of the capital with its preparations and press conferences stood in sharp relief against the dead silence or deadly gunfire in much of the countryside and some smaller cities.

The Reagan administration, the Salvadoran armed forces and, at least publicly, the Christian Democrats have opposed substantive talks with the guerrillas, despite repeated offers in recent months by the insurgents to negotiate without prior conditions.

It is not clear whether Christian Democratic President Jose Napoleon Duarte now favors negotiations, despite such thinking among some of his senior colleagues in the party. Duarte said at a press conference this week that there would be no negotiations with the left, but, as one Christian Democrat suggested, the issue is so explosive that it would be virtually impossible for any politician publicly to endorse talks at this moment.

But the increasing talk about the elections as a step toward a negotiated settlement reflects both the hoped-for strengths and the serious weaknesses of the process that climaxes with the voting.

Washington and the military-civilian coalition it helps sustain originally heralded the election as the only political solution to the civil war that threatens the stability of the region.

The government's position is that the Marxist-led insurgents cannot be trusted to keep any agreement and should not be allowed to win at the negotiating table what they could not win on the battlefield or at the ballot box.

"The armed forces are not disposed to negotiate on any level," said the defense minister, Gen. Jose Guillermo Garcia, who is considered the most powerful individual in the country. He told a press conference today, "The only negotiation we accept is the word of the people and they will speak tomorrow."

A major objection to talks, posed privately by some U.S. diplomats and Salvadoran officials, is that the current coalition is too divided to negotiate effectively.

"We were in a very weak position," said one leading Christian Democrat, "too weak to talk to the left since we had the right against us also. There will be a greater possibility of talking if all the right is together in the constitutuent assembly," he continued. "An agreement will have to be reached to talk to the left and find a way out."

Another high-ranking Christian Democrat talked of formation of a consensus for talks in the constituent assembly, even if changes in the upper ranks of the party are necessary because some of its leaders were perceived to have been "burned" in the eyes of both the left and the right through their actions in the current government.

Foreign Minister Fidel Chavez Mena is commonly rumored to be the rising star in the party because of his ability to get along with both the conservative private sector and liberal and leftist figures here and abroad.

U.S. Ambassador Deane R. Hinton, meanwhile, has been leaving an ever wider crack in the door to negotiations by suggesting repeatedly that Washington and the government that emerges from the elections should be "flexible" in approaching the subject.

All this comes in the context of a war that appears increasingly costly for the Reagan administration both in terms of its material support of this country and in the political price of serious opposition to this government in the United States and abroad.

In recent weeks, even as Washington's rhetoric has appeared most bellicose, initiatives have been taken to renew dialogue with Nicaragua and Cuba, both of which are considered key actors on the side of the Salvadoran guerrillas.

The original hope of Washington and the government here was that the electoral process would give the current government, which came to power through a 1979 coup, much-needed international legitimacy; that it would isolate the radical left and insurgents, who would not participate; and that it might give moderate civilians, especially the Christian Democrats, more authority over a military apparatus that often seemed out of control.

Despite all the hopes and expectations being placed in the election here and outside the country, it may result in a weaker civilian rule, with juridical legitimacy but little of the international approval and recognition considered necessary to pursue the war against the guerrillas or to talk with them.

In addition to the Christian Democrats there are five other parties fielding candidates for the seats in the assembly. All of them are ideologically to the right of Duarte's party. Three are expected to show significant results in the voting.

The most extreme opposition comes from the Nationalist Republican Alliance, which has emerged in less than a year to become the principal competition for the Christian Democrats through organization, slick campaign style and especially a charismatic, highly controversial leader in ex-major Roberto D'Aubuisson.

The next most powerful party, the National Conciliation Party, served as the mechanism for the rule of a small economic and military elite until the October 1979 coup that essentially brought the current government to power.

A group of relatively moderate, essentially middle-class conservatives in the Democratic Action Party are generally thought likely to win a few seats in some urban areas.

The campaigns have been swamped in vitriol.

El Salvador has a tradition of elections but no tradition of democracy. Presidents often take power by force or fraud.

Tens of thousands of people have died in political violence during the last three years. When the ban on political campaigning was dropped at the beginning of the year, bitter hatreds erupted in the pages of the newspapers and on the radio and television in political advertisements. Most parties attacked the Christian Democrats, who are facing the drawbacks of incumbency, all the blame, with few of the advantages.

While the threat of major violence by the guerrillas to shut down the elections has hung over the proceedings, the parties rarely addressed the war and ways to end it as an issue.

D'Aubuisson might seem an exception, since he repeatedly said he favors an all-out military solution at whatever price. But his denunciations of the insurgents were always tied to his denunciations of the Christian Democrats. He says they are all communists.

The Christian Democrats portrayed D'Aubuisson, on the basis of circumstantial evidence, as the leader of right-wing death squads.

If the Christian Democrats do not win an absolute majority of the seats in the assembly, and few people expect they will, they will be forced to form a coalition, probably with Democratic Action, but possibly with National Conciliation. D'Aubuisson's party will have the same option and despite the talk in both parties of a "national unity," the differences seem irreconciliable.

"We could not get together with D'Aubuisson," said Julio Adolfo Rey Prendes, a Christian Democratic ex-mayor of San Salvador. "We are too different."