For 35 years, the Somoza dynasty's man in Washington, Ambassador Guillermo Sevilla Sacasa, has basked in the warmth that the U.S. government radiates upon the stable allies. But with Nicaragua now convulsed, its courtly, portly dean of the diplomatic corps confronts increasing signs here, too, that the party is over.

In his decades of extraordinary diplomatic endeavor, Sevilla Sacasa has served an unprecedented 20 years as protocol chief of the Washington ambassadors, gathered 50 international decorations and a library full of photos with fulsome inscriptions from seven U.S. presidents, collected a title from his nemesis columinst Jack Anderson, as "the most pompous person in town" and gained a universal respect for his memory and stamina on the cocktail circuit.

Now 70, Sevilla Sacasa is winding down his career in lockstep with President Somoza, whose family he has represented since 1943.

By his own account, they were good years. According to his opposition in the often histrionic debate at the Organization of American States, they were servile. To those who knew him and his country best, they were years of semi-exile, at the service of a family that barely trusted him.

As the stability that was once the forte of Somoza rule crumbles, this loquacious pug of the OAS is forced onto the defensive as never before,raising one last time the old specter of communists at the gate - not in Guatemala this time, nor in Cuba, but in Nicaragua itself among the ever more credible opposition, the Sandinista guerrillas.

It is perhaps the ultimate irony of this complex career that Sevilla Sacasa began it as a fervent Sandinista himself - a determined opponent of the first Anastasio Somoza, the founder the dynastry.

Somoza, the founder of the dynastry.

Sevilla Sacasa's conversion to defender of the dynasty came about by marriage to Lillian, sister of the current president, Anastasio Somoza Jr., and daughter of the dynasty's founder. His critics say it was an expedient marriage. Indeed, the wedding present from the father of the bride was the embassy in Washington.

Others say it was a marriage of love. It is certainly true that Sevilla Sacasa gave up a career as lawyer and politician with at least as much promise as the ambassadorship has provided. And rather than the Somozas accepting him as a welcome in-law, they have become estranged from sister Lillian, his wife.

In this light, Sevilla Sacasa's dedication seems less to the Somoza family, as his critics allege, or to the people of Nicaragua, as he is wont to contend, than to enhancement of the title that has become his own - dean of the diplomatic corps. And hence, of the diplomatic corps. And hence, ment as well. Warm Words

"I love the United States, I believe in the United States, I treasure the United States." Sevilla-Sacasa was holding Forth at the million-dollar Nicaraguan residence on Ellicott Street NW. It was a party for a nephew of Somoza whom the Sandinistas had held hostage during the takeover of the legislative palace in August.

Sevilla-Sacasa was surrounded by the gallery of presidential portraits with warm inscriptions suggesting his his affinity for the United States was requited. There are inscriptions from Franklin D. Roosevelt, Harry S. Truman, and Dwight D. Eisenhower; there is a letter from Jacqueline Kennedy in 1961: "May you be Dean for twenty more . . . " (she signed for her husband, too) and from the Lyndon Baines Johnson, on a photograph standing beside a statuette of LBJ, "To the Dean from all of us with appreciation." Richard Nixon, Gerald Ford and President Carter are represented as well, and even J. Edgar Hoover's visage is on the piano. "He was a neighbor and a friend," said the ambassador. An Uncle Ousted

Sevilla-Sacasa's birthplace, Leon, has been described by one of his contemporaries as "the Boston of Nicaragua." It now is shattered by the outburst of civil war in September but in the dean's early days Leon was a proper place to study law. He quickly became a fixture because of his brilliance.

In those early years, the U.S. Marines dominated Sevilla-Sacasa's turbulent country. They came to assure collection of debts and overstayed whatever welcome they had. Before departing in 1933, the Marines supervised an apparently fair election in which an uncle of Sevilla-Sacasa won the presidency. In 1934, the future dean took a seat in the Congress, representing his uncle's partly.

That same year, the leader of a guerrila group that had fought against the Marines, Augusto Cesar Sardino, was coaxed down from the hills to sign a peace pact. However, the National Guard that the Marines had organized to maintain order assassinated him. The Guard was under the command of Gen. Anastasio Somoza, who had been appointed by the Marines.

Sevilla-Sacasa became a leading spokesman in the Congress for those who rallied to the cause of the martyred Sandino. In 1936, Gen. Somoza threw out Sevilla-Sacasa's uncle and seized power. Diplomat in Limbo

Lillian Somoza was considered a beauty. She had attended Gunston Hall School for Girls and in 1940 she was chosen queen of the Apple Blossom Festival in Winchester, Va.

On her return to Nicaragua, Sevilla-Cacasa courted and won her despite opposition from members of her family and probably from his as well. But they were determined, and Gen. Somoza provided a wedding in 1943, that is still talked about. They came to Washingthat August.

At that time, Sevilla-Sacasa did not speak English - some who become lost in his thick accent maintain that he still cannot. Initially, then, it was Lillian Somoza Sacasa, with the requisite English, a quick mind and the confidence of her father, who represented Nicaragua while Ambassador Sevilla-Sacasa learned his way.

In 1946, the dictator Somoza tried one of his experiments in delegation of the presidency that almost nipped Sevilla-Sacasa's budding seniority in Washington. Sonoza's choice for the presidency, Leonardo Arguello, proved so independent upon taking office that the Somozas had to oust their "puppet" in a coup. All this left Sevilla-Sacasa in diplomatic limbo during an extended period in which the United States recognized no government that he represented.

Sevilla-Sacasa's sedulousness won the day.President Truman's mother was ill at that point and the Nicaraguan sent flowers to Missouri. Back came a thank-you note from the president's wife addressed to "Mr. Ambassador." In what became known as the case of "bloomer diplomacy," Sevilla-Sacasa later presented this evidence to the State Department as proof of his continuity.

By 1948 the Pan American Union had evolved into the OAS, where Sevilla-Sacasa found a forum for his eloquence in Spanish. He was always been ambassador to the OAS as well as the White House. For years. Customary Courtesies

The ambassador became dean of the corps on Jan. 1, 1958, succeeding Norwegian Wilhelm Munthe de Morgenstierne and changing the license tag on the embassy's Cadillac from No. 2 to No. 1. He was now outranked only by the U.S. president, vice president, speaker of the house and chief justice in Washington's protocol order.

That meant a lot of party going. He missed few, although his wife soon retired from all but the White House functions and concentrated instead on bringing up their nine children.

Sevilla-Sacasa now served as toast-master for Washington's ever-growing diplomatic corps. He had other tasks, too, such as picking out and presenting the gifts - usually silver - that the legations collectively give on occasion. Those exercises often lodged in one society page or another.

But whatever potential the deanship had for lending serious voice to concerns of the diplomatic corps largely was lost in what a typically embarrassed, and bored, listener described as "his rambling, flowery, old-fashioned speeches."

Still, the Americans seem to have found him preferable, at least until now, to the ambassador second in longevity: Anatoliy Dobrynin of the Soviet Union.

For all Sevilla-Sacasa's anticommunist fervor in the Caribbean: he assidouously attended to the Soviets in his diplomatic rounds. Indeed, this attention to the niceties even with his enemies has been his hallmark.

Luis Reque, the longtime executive secretary of the OAS Human Rights Commission, recalls that Sevilla-Sacasa was among a long line of ambassadors who sought to have him sacked for pressing the rights issue. In the mid-60s, Reque had sought to schedule a seminar by the Commission in Nicaragua on the right to free choice of one's leaders - this at a time when Anastasio Somoza Jr., then National Guard chief, was about to conduct presidential elections.

Sevilla-Sacasa rejected the proposal and a later requested for a Commissoin visit to Managua - and even mustered the votes in the OAS Council to forbid publication of the correspondence.

Reque later learned that the dean had gone to the OAS leadership seeking his dismissal for exceeding his mandate in the rights post. Nevertheless, on the next Day of St. Louis, the Bolivian Reque's namesake, Sevilla-Sacasa made his customary call to offer felicitations.

The dean's list of calls on birthdays and saints' days could have made up a social register, with a special Latin annex, and as far as is known he kept it in his head. 'Yes, my general'

Somaza the elder was assassinated in 1956. His eldest son, Luis, served a term in the presidency and died afterward of a heart attack. In 1967, it was the turn of Anastasio Jr., the West Point graduate. During the runup for that election, an audacious attempted coup against the dynasty failed utterly. It caused little stir in this country, although some observers criticized as brutal the crushing of the plotters by the National Guard.

What took the play away from the plot was a party the night before, with President Johnson, Vice President Hubert Humphrey and Secretary of State Rusk attending festivities for the wedding of a daughter of Sevilla Sacasa. As a columnist pointed out at the time, "To Nicaraguans, this was tacit endorsement of the Somozas, allied in politics and marriage with Sevilla Sacasa."

When Nicaragua's neigbors, Honduras and El Savador, went to war in 1969, ostensibly over a disputed soccer match, the OAS dispatched the dean as peacemaker.

Sevilla Sacasa, with a planeful of mediators, flew into San Salvador to meet with the ruling colonels in the main dining hall of the imposing Intercontinental Hotel - darkened and hot because the electricity had been cut to provide instant air-raid protection.

Phone service was erratic, too, but one call came through - from an angry Somoza in Managuan to the dean. While the sweating officials pretended not to evaesdrop, he shouted, "Yes, my general" and "But you see, my general . . . " for 45 minutes over the bad connection while Somoza dictated his concept of Central American power politics. No one other than Somoza is known to have put the dean at a loss for words in Spanish.In any case, the peace was soon achieved and has held there since. Night in the Shrubs

The new generation of Sandinistas gave Sevilla Sacasa a taste of things to come four years ago when the guerrillas raided a Christmas party for U.S. ambassador Turner Shelton in Managua, which the dean had flown down to attend.

By the account of Guillermo Lang, Nicaragua's ambassador to the United Nations, the dean fled to the garden when the raid commenced and ducked under the shrubbery. He spent the entire night with the bougainvillea, only to be discovered by the guerrillas in the morning.

Undaunted, he took up the bargaining with his captors that led to a deal. "The sum of money initially demanded was reduced substantially" by his efforts, the dean said back in Washington.He offered this advice to others who might fall into such a fix: "Personal valor is one of the weapons of diplomacy."

The outbreak of civil war in September saw the dean soundly defeated on his home court, the OAS Council. With clashes between the Nicaraguan Guard and the rebels spreading through every city and spilling into Costa Rica next door. Venezuela called for the OAS to meet under the hemisphere's mutual security treaty.

"I declare that this request of the government of Venezuela is a condemnable request . . . that destroys our organization's principle of nomintervention," declared the dean. There followed an exchange of insults with Venezuelan Amabassador Jose Machin.

Outraged debate between the dean and Machin has become a symbol of the Pan American Union, supplanting the parrots that used to squawk in the lobby. More than once Sevilla-Sacasa has challenged the Venezuelan, half his age, to fight it out physically.

Venezuela is South America's premier democracy these days, the antithesis of Nicaragua's dynastic longevity. But among the more treasured decorations on the dean's chest is the Venezuelan Order of the Liberator, Simon Bolivar, given during dictatorial days in Machin's own country before it turned to constitutional rule 20 years ago.

On the day of the crucial vote last month to condemn Nicaragua, Machin was joined by Cost Rica Ambassador Jose Rafael Echeverria in the commandos wit the defending dean.

Echeverria, demanding the loss of life at the hands of Somoza's National Guard, asked Sevilla-Sacasa, "What is more important, to defend one family or to defend many families?" The dean challenged Echeversia to affirm whether "these 'families' are terrorists."

It went on for hours until U.S. Ambassador Gale McGae took the floor to declare: "I am frankly fed up and tired of listening to what they have to say of each other." No one suggested that the American was impolite.

The OAS invoked the security treaty against Nicaragua and voted "to censure and deplore" it for the incursions into Costa Rica. Red in the Rainbow

Since then, the dean has gone about his ample business. He attended the installation this year of both popes (wearing a decoration presented him in 1962 by Pope John XXIII). Indeed, as with parties in Washington, he has missed few international gatherings.

Sevilla-Sacasa has also been called at least once to Managua for consultions. But Somoza has done much of his diplomacy in this country, as usual, through his network of freinds, business associates, West Point classmates and hired lobbyists rather than his embassy.

The Justice Deparment's registry of lobbyists for foreign interests lists 10 as having worked for Nicaragua in recent years, five of them still active.

MacKennie McCheyne of Washington, also known as the Nicaragua Government Information Service, is the biggest spender, registering receipts of of $311,000 from Nicaragua in 1977 for public relations services.

Two other Washington representatives register miscellaneous contributions for political campaigns, usually $100 apiece. The firms are Creamer, Visser, Lipsen and Smith, and Kerth and Korth.

Sevilla-Sacasa has never complained pubicly that Somoza has chosen to keep his own lines to the United States, but certainly has added to the impression that the dean deals more in pomp than substance.

"I consider that I am a very honest man, a clever man," he said at his party after the palace raid. He also assured that Somoza would serve out his term in 1981 and that he, the dean, would not retire to Managua.

We had spent a large portion of the evening entertaining a young woman and her businessman husband. He asked the waiter for a glass of gin and declared, a bit wearily.

"When I came here the rainbow had only seven colors. Now it has 11." It was one of those statements that make people wonder if he has mastered English. He elaborated: "The red now has several colors. It has daughters and granddaughters."

One thing was clear. He meant times have changed. Apparently he also meant that what he always saw as the communist menace is more complex now. It was late. The party was over.