When ex-major Roberto D'Aubuisson went to his military school's class reunion last November after a clandestine year and a half as a fugitive coup plotter, he had every reason to think he would find support there for the ultra-rightist political party he had just founded.

In El Salvador's military-dominated politics, few bonds are stronger than those formed among Army officers in their four grueling years at the General Gerardo Barrios Military School.

The military's clannishness had protected him when civilian politicians and even other officers had tried to arrest him. There is within the Salvadoran armed forces a sense of duty to one's comrades that a senior U.S. diplomat once likened to the Mafia's rule that one never betrays a member of "the family."

But what D'Aubuisson discovered is that while his class was becoming a key force in the Army, and hence one of the most potent political powers in the country, he was no longer, quite, part of it.

"We are not any party's institution. We, the Army, are more eternal than any party," one senior member of the class said as he recalled the evening with his retired, right-wing friend. "D'Aubuisson, even though he is a classmate, is in another dimension. He abandoned the military to be a politician, while we are here in the trenches."

The division between D'Aubuisson and much of the rest of his class reflects one of several simmering just beneath the superficial unity of the Army: splits among rightists, leftists, centrists, political neutralists; among administrators, battlefield commanders and intelligence officers; between the corrupt and the clean, counterinsurgency specialists and advocates of conventional warfare, senior officers reluctant to retire and junior ones hungry for promotion.

The men who graduated from military school in 1963 and 1964 along with D'Aubuisson have celebrated reunions as a single class or "promotion" since 1965. They have served as godfathers for each others' children, worked and studied together, helped pull each other up through the ranks.

In the 20 years since their school days--the days of the Alliance for Progress and the first wave of fear provoked by Castro's Cuba--they have seen some members disgraced and others die for politicians and coup plotters. D'Aubuisson joined the radical right. Another classmate defected to the guerrillas.

But in the last two years of civil war, a tightknit core has ascended the chain of command to become lieutenant colonels in charge of more than 5,000 troops in key units, including all three special counterinsurgency battalions trained by American advisers.

This core of officers, along with the more senior armed forces chief of staff, Col. Rafael Flores Lima, is clearly favored by the U.S. Embassy.

Flores Lima and these men seem to share with the Pentagon a basic view of the Army's role in society and a basic notion of how to fight guerrillas: they temper fierce anticommunism with moderate reformism. They reputedly are relatively free of corruption and have not been publicly tainted by connections with the death squads that attack alleged leftists.

These officers frequently join their troops on the battlefield and they are partisans of the small-unit tactics, strongly advocated by Washington as the key to military success here, which are intended to turn the guerrilla's methods against him.

When U.S. advisers have been consulted they have helped these men advance. The idea, said the analyst, is to make the Army a neutral, stabilizing force interested more in fighting effectively than plotting coups and backing politicians.

But a high-ranking civilian member of the government says the United States is cultivating Flores Lima and certain other officers to take over the government in case the current experiment with democracy fails to gain U.S. aid and stop guerrilla advances.

The Americans "play that way," said the somewhat exasperated official. "They always have the second-best solution ready. Why don't they stick to the first, best solutions, according to principles?"

As D'Aubuisson ascended to power through the March elections and became president of the new Constituent Assembly, his political confrontation with his former institution intensified.

One of D'Aubuisson's old classmates recalls a meeting D'Aubuisson and another leader of his party, businessman Hugo Barrera, had with several officers shortly before the elections. "Arrogant," was the main word the officer used to describe them. "We were not impressed," the officer concluded.

D'Aubuisson got the assembly presidency on the basis of popular votes, but the interim presidency of the country went to the Army's choice, Alvaro Magana, a civilian banker whose ties with the military date back to his high school track team, when he trained with the boys from Gerardo Barrios.

One of the first initiatives of the rightist-dominated assembly was to modify El Salvador's agrarian reform. This helped provoke a potential $100 million military aid cutoff in the U.S. Congress. One of D'Aubuisson's front-line commander school chums said bitterly, "The assembly is going to put us into a dead end with no way out."

The agrarian reform is seen by these officers not only as a key to more U.S. aid and needed political support among the peasants but as a declaration of independence from the old landed oligarchy. They remember the rich as using the Army for their own ends, making senior officers wealthy but at the same time often making them feel, somehow, like second-class citizens.

Lt. Col. Sigifredo Ochoa, an ambitious officer in the class of 1963-64 credited with cleaning out rebel concentrations in Cabanas Department (province) and often cited by U.S. officials as the most successful regional commander, once had a reputation as a right-winger. But he now speaks bitterly of El Salvador's ultraconservative rich and their allies in the assembly.

"We don't want to go back to being anybody's instrument," Ochoa said recently. "They had better understand that we are not going to be compromised. The 'gentlemen' of the right live peacefully in Miami, while we military men are here, knowing we can be killed and knowing we have to fight."

All the tensions in the military are exacerbated as the defense minister, Gen. Jose Guillermo Garcia, nears completion of 30 years in the Army. Garcia is considered the most powerful man in the country. His political skill in dealing with the conflicting currents of the armed forces has enabled him to retain his post since the October 1979 left-leaning military coup while three governments have since fallen around him.

But three decades is traditionally the limit for any officer on active duty. If Garcia and his own classmates, now the most senior officers in the service, refuse to step aside, they will be following ominous precedents. While the need for reform has provided the rationale for many past coups in El Salvador, young officers anxious to push aside a top-heavy high command have usually provided the necessary push, according to civilians with ties to the Army.

If Garcia retires--as rumored--to run as a party's candidate for the presidency, following the assembly's adoption of a constitution, he too can expect to be distanced from his vital uniformed consituency. Some Salvadoran officials see Garica's power waning already.

"Whether for good or bad," said one high-ranking member of the present government, "Garcia has been subject to a process of power loss over the last three years."