The general secretary of the Socialist Workers' Party of Spain sat in shirtsleeves in his office looking, talking and acting as if he had been the winner in last week's parliamentary elections.

Felipe Gonzalez, 35, was ebullient about his party's performance - 118 seats in the Chamber of Deputies with 28.73 per cent of the vote.

"No Matter how you examine the election results, PSOE is Spain's biggest single party," he said. "It's also Spain's oldest party - nearly a hundred years old. And Western Europe's oldest Socialist party. That was important in the election, and it's going to be important when Parliament meets to write a new democratic constitution."

Gonzalez remarked that Premier Adolfo Suarez is having trouble with the Center Deemocratic Union, the moderate coalition that won 166 chamber seats and 105 Senate seats with 38.86 per cent of the vote. The 12 parties in the coalition that rode to victory on the premier's coattails are known to be fighting over ministeries and patronage.

The numbers give the premier enough deputies to dominate the 350-seat lower house, but to govern Suarez must make deals not only with his "12 restless tribes" but with the combined left. The left won a majority of the votes but did not reap the deputies. For many Spaniards, the split between center-right and the left approximates that of the 1936 election, five months before the outbreak of the three-year Civil War. PSOE was Spain's largest prewar political force.

The usually tieless Socialist chief is not willing to form a coalition government with Suarez. He does not quite say so, but he is aware that it would be political suicide - "unless there is a national emergency. In the U.S. Republicans and Democrats don't form coalitions."

His immediate concern is to demand early municipal elections, which he is sure his party will win. He wants to force the premier to hold them "by the end of the year."

Gonzalez' party is not going to press for parliamentary elections after the new democratic constitution is adopted to replace the authoritarian charter imposed by the late dictator Francisco Franco. "They'll come inevitably within the next 18 months to two years," Gonzalez declared. "We'll win them - or try, at least."

The general secretary - who spent most of his life as an underground Socialist leader chased by Franco's police - enjoys being the head of the opposition. After all, his party was illegal until earlier this year.

"We're now part of the power structure and we are going to deal from our position," Gonzalez said. "We'll fight for our economic program, for fiscal reform, for fair wages and prices."

He has been congratulated by Suarez, who told him over the phone that "We must get together soon." The American embassy has given "Gonzalez and his party its best wishes - even though some U.S. officials worry about radicals in the party and about its Marxist and republican stand.

He dismisses their concern. "We're not going to follow the French model of unity with the Communists," he said. "We're not going to follow the West German model. We are going to follow a Spanish model. This means that we are going to have to deal with Spain as it is, and follow our program."

He made it clear that the party will not form any common front with Santiago Carrillo's Communist Party, which won 19 Chamber seats. What he would like to do, however, is to merge with the Popular Socialists, who elected six deputies. The fusion, he predicted, "is inevitable."

One of the main concerns of American diplomats is Gonzalez' attitude toward NATO and the U.S. bases in Spain. He explained that he is opposed to the bilateral treaty with the United States and will demand that the first freely elected Parliament in 41 years reconsider a pact made without the consent of the Spanish people.

"I would prefer Spain in the North Atlantic Treaty alliance instead of the bilateral treaty," he said. But the Socialists would make certain conditions, among them withdrawal of U.S. forces from Torrejon, the big base in Madrid's outskirts shared jointly by Spain and the United States.

Gonzalez explained his point of view and that of his party on all issues concerning U.S. military and financial interests in Spain to Ambassador Wells Stabler before the election. He has gone through similar exercises with Western European leaders in his trips abroad and in contacts with embassies here.

Not too long ago Gonzalez sat down with King Juan Carlos and gave the young monarch a rundown of his party's aims and goals for a democratic Spain. He assured the 39-year-old king that the Socialists - even though Republican - would not make an issue of the monarchy in the new charter.

"My talk with the king was a monologue," said Gonzales. "I talked. He listened."

He has been invited to the king's official birthday celebration Friday, but he's not sure that he is going to accept because he must go in formal dress. "The decision is up to the party," he added.

Many in Spain were concerned that a Gonzalez victory in the parliamentary elections would have sparked a military coup and brought foreign investment in Spain to a total standstill. Gonzalez never really considered a victory, even though he campaigned to win.

The proportional electoral system adopted by the government favored Suarez. So did the government machinery. The surprise was the Socialists unexpected strong showing in the Basque region and in Seville, and that it managed to elect 10 women to the lower house.

But its strength was in the urban centers of Madrid, Barcelona, Valencia and Seville.

"We're going to have to negotiate with Suarez," he said.

He is quite aware that the premier does not know most of the deputies and senators elected by the eight-week-old Center Democrats, and that each of the 12 parties that backed Suarez has a different political and economic platform. Gonzalez recognizes that this is why the premier has postponed forming a government.

Gonzalez has no such problem - he knows almost all of his party's leaders. He was instrumental in organizing the party while it was still illegal. His campaign in a private jet reinforced his leadership and gave the party a homogenous image.

"Sure we're Marxist," he said. "But what's a Marxist today? I assure you that Karl Marx wouldn't be a Marxist today.