Shaking Salvador's political equation. As left-wing rebel leaders gingerly test their wings in El Salvador, the extreme right wing is finding that it may have to answer for past actions.

For more than seven years, El Salvador's political landscape has been dominated by two rock-solid realities: the political immunity of the extreme right wing and its rampant death squads; and the absence of left-wing political activity within the country. In just two days, these cornerstones of conflict have been shaken for the first time.

On Monday afternoon, President Jos'e Napol'eon Duarte unveiled testimony implicating former Maj. Roberto D'Aubuisson, leader of the far-right National Republican Alliance, in the 1980 assassination of Roman Catholic Archbishop Oscar Arnulfo Romero.

Two hours later, Guillermo Ungo - self-exiled president of the Democratic Revolutionary Front - returned after seven years to explore political openings on the left.

Neither the return of rebel politicians nor the reproach against right-wing extremists is expected to have immediate effects. But now that past assumptions have been shattered, analysts say, the Salvadorean conflict seems to be poised at a crossroads, ready to turn either more dangerous or more docile. While the prospects for broader political dialogue may have heightened, they say, so has the possibility for right-wing retaliation.

President Duarte's indirect accusation of Mr. D'Aubuisson in the archbishop's killing caught many diplomats and politicians here by surprise. Not because D'Aubuisson has a squeakyclean image: His role as ``intellectual author'' in this murder plot and many others has long been suspected. But few analysts thought Mr. Duarte would finally stand up to Army abuses and right-wing hit squads.

``This doesn't necessarily mean that Duarte is going after D'Aubuisson,'' says Robert White, former United States Ambassador to El Salvador. ``I doubt whether they have enough witnesses to nail him.'' But ``by naming D'Aubuisson, [Duarte] is trying to take the Salvadorean people through some sort of catharsis, ... making him a metaphor or symbol for all the unsigned crimes.''

At the time of his murder on March 24, 1980, Romero had become a symbol of the public's growing disenchantment with the Army. His death polarized the country, and the failure to find his killers confirmed suspicions that Duarte's hands were tied by the extreme right.

But it appears that the far right is no longer untouchable. According to Duarte, Amado Antonio Garay, who drove the assassin to the church where Romero was giving mass, has come forward with new, though unsubstantiated, charges. Mr. Garay claims that three days after the rifle shots ripped the silence of morning mass, he drove his boss, Capt. Alvaro Saravia, to D'Aubuisson's house. The captain told D'Aubuisson of a hit man's exploits: ``We did it just as you ordered it.''

Captain Saravia was detained by US authorities in Miami Monday on immigration violations. Duarte has said he will request Saravia's extradition.

Asked whether this meant that D'Aubuisson - Duarte's chief rival in the 1984 elections - masterminded the murder, Duarte replied: ``That's the conclusion you can draw....''

D'Aubuisson, who was outside El Salvador at the time of the murder, says the charges were concocted to distract attention from the war, the economic crisis, and the arrival of rebel leaders Ungo and Rub'en Zamora.

Rebel leaders agree that the release of new information was designed to deflate the importance of their historic return.

``There's too much coincidence that after seven years, precisely on the day that we are arriving, that this is discovered,'' said Ungo, asserting the archbishop's killers had been known from the beginning. ``But apart from that, I'm glad. I hope they go to the bottom of the problem.'' (After joining Mr. Zamora, vice-president of the Democratic Revolutionary Front (FDR), who arrived Saturday, Mr. Ungo proposed restarting government-rebel talks on Dec. 5.)

But Zamora, in a brief interview Monday night, expressed fear that the revelations would only antagonize right-wing extremists at a time when they were already angry about the rebels' political presence in San Salvador. ``The extreme right must be furious now,'' he said. ``That worries me.''

It was not the only disturbing omen for the returning rebels. Duarte is assuming a hard-line posture. He issued a veiled threat to Ungo and Zamora: if they don't renounce their links with the Marxist-led Farabundo Mart'i National Liberation Front, he said, then they can be held legally responsible for any violent actions by the front.

So far, a palpable fear has hampered rebel leaders' efforts to open political space. Reni Rold'an, president of the Social Democratic Party, has received three death threats in 10 days, apparently because of his connections with Ungo and Zamora. Attendance at a union meeting with Zamora was low, since people feared associating with the FDR, union officials said.

Despite the fear, however, the mere presence of Ungo and Zamora - and the implication of D'Aubuisson in Romero's death - show that the situation in El Salvador has been fundamentally altered.