Since the 1974 collapse of its military dictatorship and the reestablishment of democracy, Greece has enjoyed a long spell of political stability under moderate leader Constantine Karamanlis that contrasts sharply with confusion and turmoil in neighboring countries such as Italy, Yugoslavia and, most recently, Turkey.

But all this may soon end. Worsening economic conditions, mounting pressure for social change and nagging foreign policy problems appear likely to usher in a period of political uncertainty.

"We are quickly approaching the end of the Karamanlis era and of one-party government by the right," a respected Marxist politician said over coffee recently. Although Greeks of other political persuasions might phrase it differently, there is a general consensus that next year's election, the first since 1977, will probably be a watershed in postdictatorship Greek politics.

Although elections, which must be held by November 1981, are still a good way off, there are indications that the ruling New Democracy party is losing ground sharply and that Pasok, the strongly anti-NATO Panhellenic Socialist Movement headed by Andreas Papandreou, will make substantial gains.

The implications of a Pasok victory are considerable, since Greece and its islands -- particularly Crete, where two important U.S. bases are located -- are of prime strategic importance for the Atlantic Alliance.

Many informed Greeks are convinced that neither of the two major parties will win a majority, opening the door to coalition government for the first time in six years.

Other than the possibility of a grand coalition, one prospective partner for either group would be the remnants of the Democratic Center, which disintegrated after the last elections but which may have trouble surviving in an electoral system that penalizes groups with less than 17 percent of the vote.

The other partner would be the pro-Moscow Communist Party of Greece-Exterior, which shares Pasok's anti-NATO bent and expects to increase its percent 9 percent of the vote.

"A repeat of the 1967 coup is unthinkable," said publisher Helen Vlachos, a liberal opposed to the junta who after the takeover shut down her newspaper, fled house arrest and went into exile. "The Army has been inoculated against intervention and thinks only of winning the people's respect."

But several left-of-center politicians said they doubt that the military would tolerate a governing alliance between Pasok and the Communists. Not only were 2,000 officers trained during the seven-year dictatorship, but the Greek civil war with its 120,000 dead has left a backlog of anti-communist bitterness 30 years later.

With the exception of the dictatorship, Greek politics have been dominated by the right since the civil war and there is no doubt that the stability provided by Karamanlis' rule in the late 1950 and early 1960s provided the country with what one Greek journalist terms "a golden period" that until recently gave Greece an average annual growth rate of 7 percent.

Since the collapse of the junta in 1974, Karamanlis' party, New Democracy, has consistently won a majority of the votes in the 300-member single house parliament.

That majority, however, has shrunk steadily since 1974, when Karamanlis returned from 11 years of exile in France to lead the party to a smashing victory of 54 percent of the popular vote. In 1977 the party received 42 percent, enough to win 175 seats, but the outlook is not bright.

Last April Karamanlis, 73, resigned as prime minister and was elected president of the republic. Some Greeks believe he wanted to help his party develop new leadership. He may also have been enticed by clauses of the 1975 constitution that give the president important powers reassuring to those who view the aging politician as a guarantor of Greek democracy.

His successor, Prime Minister George Rallis, is a pragmatic man without Karamanlis' charisma who, said a Greek businessman who supports New Democracy, "will have trouble leading "his party to victory at the polls. Rallis, 62, whose father and grandfather both also served as prime minister, has internal party troubles as well.

In contrast, Pasok's fortune's appear to be on the rise. In 1977 Papandreau's party doubled its votes to win 25 percent of those cast and 93 seats in parliament.

"Businessmen I know in the north tell me Papandreou would win hands down if elections were held today," said a key figure in the Greek shipping industry who believes a major reason for the socialist party's success has been "the major crisis of confidence toward the West" that followed the failure of Greece and its allies to keep the Turks out of Cyprus in 1974.

Greek-Turkish tensions over Cyprus exploded in the summer of 1974 when a Turkish invasion of the heavily Greek-populated island followed a coup reportedly inspired by the Greek military junta, which collapsed shortly thereafter. The Greeks reacted bitterly to American and allied acceptance of a situation that forced many thousands of Greek Cypriots to leave their homes and left the Turks in control of 40 percent of the island.

Greeks of all views are also still quick to express resentment for the tolerance that its NATO allies, particularly the United States, showed toward the military dictatorship.

This resentment was so strong that even Karamanlis, a pro-Western politician whose major foreign policy has been sponsorship of Greece's coming membership in the European Common Market, was forced to take a stand. In August 1974 Greece withdrew from NATO's military structure. Today the Greek government is seeking to negotiate its reentry into NATO on terms, strongly opposed by the Turks, that would give it the same degree of operational control in the Aegean that it had in 1974.

But it has been forced to recognize the anti-NATO, anti-American sentiment that is reflected in Pasok's growing popular appeal.

Convinced that U.S. policy continues to tilt toward Turkey, Rallis has now linked the fate of U.S. bases in Greece to the successful outcome of the NATO negotiations.

In addition to its nationalistic emphasis, "Pasok is for the working man," said George Koutouzis, a headwaiter at an Athens hotel. Like many Greeks he complains bitterly about the government's failure to deal effectively with 25 percent inflation. Strongly anticommunist, Koutouzis said if Pasok becomes Greece's number one party after the next elections, "finally there will be something for us."