The political party of the old landed oligarchy emerged as the key power broker in El Salvador's struggling new democracy today, effectively dooming the presidency of Christian Democratic leader Jose Napoleon Duarte.

With 85 percent of Sunday's election results tabulated, it was clear that a loose coalition of five right-wing parties have the power to dominate the coming National Assembly, and they are united chiefly in their opposition to Duarte. His hopes of forming a coalition government evaporated when the National Conciliation Party (PCN from its Spanish initials), which came in third, publicly called for him to step down and join in a government controlled by the right.

"The only consensus at this point is that Duarte must leave the presidency," said PCN secretary Raul Molina Martinez at a news conference.

Such a sharp change in political direction would have broad implications nationally, since Duarte had supported a major agrarian reform and nationalization of the banking sector that were anathema to many of those now in a position to exclude him. It would also have grave implications for relations with the United States, which strongly supported Duarte in those policies.

It was also clear, however, that Duarte's party would remain a major actor in the political drama, whether in some government of national unity controlled by the right or in opposition. "We consider ourselves the only guarantee for peace, democracy and social change in this country," said the chief of the Christian Democratic Party and its top candidate for the assembly, Julio Rey Prendes.

The election results greatly diminished chances of the eventual government negotiating with the left-wing guerrillas, according to politicians on all sides. Rightists consistently have opposed such talks and a Christian Democrat who had leaned toward a post-electoral initiative now says the massive vote precludes it.

Indeed, the civil war remained the major question mark in what otherwise looked very much like a classic democratic political power struggle.

For the first time in the modern history of the country, none of the major political actors, including the armed forces, seriously challenged the election results. Instead, the turnout of more than 1 million voters was taken by all sides as a clear rejection of the rebels' call for an election boycott, a popular demand for peace. More than a fifth of the entire population went to the polls, many dodging guerrilla bullets to do so.

Although Duarte's Christian Democratic Party now appears to have won about 41 percent of the vote, much of which was clearly a vote of confidence in him, the other five parties in the race see his failure to win a majority as a rejection of both Duarte and his calls for major reforms.

Key to the new situation is the PCN, which ran the government for the aristocracy from 1961 until it was ousted by a coup in 1979.

Initially formed in response to popular demands for liberal reforms following a 1960 military coup, the PCN chose military candidates, who were declared victorious, in four elections between 1962 and 1977. Although it initially garnered a fair range of political support, and was believed receptive to limited social and economic reforms, the PCN gradually became more right of center.

By the time its last president, Gen. Carlos Romero, was overthrown by junior officers in October 1979, control of the party was in the hands of the most conservative sectors of both the senior military establishment and the country's wealthy landowners.

With 1,030,901 votes counted, the PCN stands third in the tally with about 17.6 percent of the vote, behind the Christian Democrats and the far-right Nationalist Republican Alliance (ARENA), which has 28.7 percent. Votes nullified by defacement, blanks and other forms of possible protest balloting made up 11 percent. The balance is not expected to shift in the final tally.

Only one of the remaining three parties, all of them well right of center, did well enough to get any seats in the 60-member constituent assembly. Democratic Action should win two or three seats but this would not permit Duarte, who has 26 or 27 seats, attain the 31 seats--a majority of one--he needs to form a government. Neither can ARENA, with a projected 18-20 seats, govern without the help of the PCN, which is expected to have 12 to 15 seats.

"There exists the possibility" of a joint government in which all the parties have some role, said Dr. Armando Rodriguez Equizabal, a member of PCN's executive board, in an interview, "but any alliance with the Christian Democrats would validate all the errors they have committed."

He said the PCN would demand, beside Duarte's ouster, a rewriting of Duarte's land and banking reform policies and a guaranteed continuing struggle against the guerrillas. "We will never negotiate with them," he said. These demands are clearly agreeable to ARENA.

The PCN, he said, is not as interested in naming the provisional president that the assembly will choose next month as it is in having political control of the assembly. "First you must build the structure, then you can decide who stands upon it."

The process of setting up the assembly, picking a provisional president, writing a new constitution and then discussing whether to keep, change or discard all 1,000 decrees of the current government will take more than the year previously thought necessary, Rodriguez said. The president could well serve two or three years, he continued.

In a joint manifesto, the five rightist parties promised to "maintain and perfect" ongoing political and social reforms, to respect human rights, to promote free enterprise and to reject violence, communism "and communitarianism"--the ideologly of the Christian Democrats. The five parties have yet to form a true coalition, however.

A well-placed PCN source indicated that in spite of the rightist alliance, the party will not agree to the presidency for ARENA leader Roberto D'Aubuisson, the charismatic former National Guard major. D'Aubuisson's ascension to power was thought likely to lead the U.S. Congress to cut off crucial aid.

That leaves the most likely candidate for the provisional presidency as Rene Fortin Magana, president of the small Democratic Action Party and the current favorite. An attorney, Fortin Magana said yesterday he would not support Duarte.

At the Christian Democratic Party, Duarte's son Alejandro insisted to reporters that the party is still in charge. "We got 40 percent of the vote in 90 percent of the towns in the country," he said. "People were voting for President Duarte. That is the reality and it cannot be disregarded. We feel like winners."

Party president Rey Prendes said it has "more power than any other party" and therefore must be strongly represented in any national unity government. Otherwise, he said, the people who voted for it would lose faith in democracy, "and if you lose faith in democracy you have to find other ways to change things."

If the Christian Democrats go into opposition, he said, "God help this country." Under insistent questioning, Rey Prendes was careful not to rule out possible talks with leftist politicians allied with the guerrillas. "We have to negotiate first with the parties that have just finished participating in this election," he said.

Another party official pointed out in an interview that the Christian Democrats are by far the most disciplined and most ideologically coherent party, while the rightist groups "are united only in opposing us. We will govern even in opposition, if it comes to that, by dividing and conquering," the official said. He, too, said the debate over the assembly committee setup and political structure will be crucial. "There will be things in which a two-thirds vote will be required," he said.