While the fate of El Salvador's experiment in democracy was in the balance here today, U.S. Ambassador Deane R. Hinton offered a press conference that focused instead on some "Russian-style" grenade parts and detonators that he said exposed "support from outside" for guerrillas seeking power here.

Only reluctantly, afterward, did he discuss the shifting political kaleidoscope. This seeming nonchalance tended to reinforce a conviction about Washington's attitude toward developments here held by the right-wing parties that triumphed in last month's elections:

They believe that massive U.S. aid to combat guerrillas and keep the economy afloat will continue no matter what kind of government emerges here, because the real U.S. concern is with communism, not with El Salvador.

"The United States has never cut off aid anywhere for very long or even entirely," said a supporter of Roberto D'Aubuisson's Nationalist Republican Alliance who is also closely linked to the armed forces. "Reagan will never let the communists win here. It's just a complete bluff," he said of U.S. efforts to link continuing aid to maintenance of reforms and reinforcement of human rights.

Hinton's weapons display today had its counterpart last week from the Salvadoran armed forces, who summoned the press to see a young man and woman they said were captured guerrillas. The two turned out to be yet another sideshow, peasants who said they were drafted by force into a ragtag guerrilla group and stayed only a week before escaping. They said they worked in the rebel kitchens and knew nothing whatever about guerrilla operations.

The State Department continues to see El Salvador chiefly as the southern front in the worldwide war against Soviet communism. For the United States, it is almost as though the convoluted maneuvering of Salvadoran politicians trying to form a coherent government were the sideshow.

But for a turbulent week, the politicians have been center stage as the guerrilla war in the countryside seemed to ease. The show appears just to be warming up, and no one knows that better than Hinton.

Rightist former Army major D'Aubuisson--whose emergence as a national leader, Hinton once said, would spark a U.S. aid cutoff--emerged yesterday as president of the newly elected constituent assembly, a post of considerable potential power. His loose two-party rightist coalition holds all the assembly's important jobs so as to predominate over the minority Christian Democratic Party, if the coalition can stick together.

Much depends on the political skills of D'Aubuisson, a former intelligence officer more accustomed to planning coups--he tried three times--than planning economic policy.

But the Army also wielded its own considerable power yesterday, virtually ordering the squabbling politicians to name as interim president of the nation next week a man whom the U.S. Embassy, the Christian Democrats and the Army all like, but whom D'Aubuisson's supporters detest: banker and attorney Alvaro Magana, 57.

The presidency, although it is provisional until full elections are held under a constitution to be drawn by the assembly, also has great potential strength. The Army and the State Department hope that an independent like Magana might paper over the chasms between the Christian Democrats and the right wing and lead a strong war against the guerrillas while observing human rights to the satisfaction of the congressmen who hold the U.S. purse strings. But the legislature here under D'Aubuisson will have to pass the local laws.

The State Department went to great pains yesterday to note that the presidency of the assembly is not the presidency of the country, but the fact is that the balance of power between the two posts has yet to be determined.

So it was that Hinton seemed unwilling to answer political questions today, even though it was his first public appearance since the day after the March 28 elections. He said he "would certainly hope" that the American public is willing to continue providing economic and military aid, even with D'Aubuisson in a high post, because "he has been elected freely by the people of El Salvador."

Hinton insisted he had always intended to try to cooperate with whatever government emerged from the election. Asked, as he was trying to leave the room, about the U.S. role in bringing about a government here, Hinton seemed to choose his words carefully. "It is true that we indicated a belief that some actions could be more helpful to El Salvador and to support for aid for El Salvador, which will continue in any case, than others," he said.

What does "continue in any case" mean? Hinton did not elaborate as he was dodging reporters shouting after him down a stairwell. Embassy spokesmen said it probably meant very little.

The embassy mounted a major campaign to convince Salvadorans that Congress is not bluffing, bringing in House Majority Leader James C. Wright Jr. (D-Tex.) and, this week, Ambassador-at-large Vernon Walters to deliver the message. It came from Secretary of State Alexander M. Haig Jr.: form a government united among all three major political parties, or else.

The Army was apparently convinced and so was half of the right-wing coalition that dominates the assembly. The National Conciliation Party, which ran El Salvador from 1961 to 1979, abandoned D'Aubuisson's party to agree on Magana as national president.

When the time came to elect the leadership of the assembly itself, the two right-wing groups were together, naming D'Aubuisson chief and assigning three other major jobs and five minor positions to themselves.

The Christian Democrats, who have run El Salvador for the past 29 months in coalition with the military, were excluded from power in the assembly. But they did not seem too concerned. "We never had it anyway," said Julio Rey Prendez, the party's chief deputy.

The National Conciliation politicians were jubilant, their power-broker position having been proven beyond doubt. "The true unity of the nationalists has been reaffirmed," said Dr. Julia Castillo, a National Conciliation deputy who was elected second legislative vice president.

But it will take many more votes before the true balance of power is struck. For now, Hinton is trying to look the other way.