Socialist leader Mario Soares, who helped avert a Communist takeover in Portugal in 1975, met with highlevel U.S. oficials this week to assure them that the strategic Mediterranean ally is still a functioning-if somewhat unstable-democracy.

Soares, a former prime minister and leader of the country's largest political party, stressed in meetings here an in New York the "symbolic" importance of Portuguese democracy in Southern Europe and Latin America.

Although Soares is now out of power, both because he still has a substantial political base and because of his moderating role in the stormy Portuguese transition from 50 years of rightist dictatorship to the present constitutional democracy.

Portugal is "a relatively new democracy that very nearly went communist in 1975," said one U.S. official. "It's in our interest to see that things go well there."

Soares, this official said, "played an important part in seeing to it that Communists did not get the upper hand" in 1975.

Among those who discussed Portugal's prospects with Soares were Vice President Mondale, Secretary of State Cyrus Vance, national security adviser Zbigniew Brzezinski and senators George McGovern (D-S.D.) and Edmund Muskie (D-Maine).

Soares, 54, who has been trying to break the monopoly of Portugal's Stalinist Communist Party over the country's unions, also met with Douglas Fraser, president of the united Auto Wokers, and Lane Kirland, secretary-treasurer of the AFL-CIO.

In an interview, Soares stressed the "symbolic" importance of Portugal's transition to democracy, especially to southern European countries and in Latin America.

Sounding like a man who was enjoying the freedom of being in opposition after two years as prime minister, Soares criticized the current nonparty government but said his party would not seek an election until the regularly scheduled one in the middle of next year.

Soares was ousted by president Gen. Antonio Ramalho Eanes last Jult after his shakt coalition with the conservation Social Center Democrats collapsed. Since then the country has been ruled by two successive nonpolitical Cabinets named by the president.

The governments that followed Soares have had the unhappy job of enforcing an austerity program agreed to during the Soares' government that is designed to qualify Portual for a badly needed $50 million loan from the International Monetary Fund.

At the same time, the party closest to the Socialists in size-the Social Democrats-suffered a serious spilt when 37 of its 73 members of the National Assembly defected.

This left Soares headiing the country's largest and strongest party but with none of the responsibilty of governing.

When Soares professes a reluctance to become prime minister again, the opportunity may arise before next year. Both the Communists and the Social Democrats have said they will call for a no-confidence vote. If the president is not able to form a new government that has parliamentary support, elections will be held.

Soares expressed the fear, however, that President Eanes, a 43-year-old general with a broad following, was planning to expand the powers his office "beyond what is in the constitution." CAPTION: Picture, MARIO SOARES . . . continues as factor in Lisbon