Cheered by tens of thousands of ardent supporters in downtown Constitution Square, Socialist Prime Minister Andreas Papandreou ended his electoral campaign tonight -- and completed the first full term of office of any left-wing government in modern Greek history.

With voters set to go to the polls on Sunday, that seemingly mundane accomplishment has been overlooked in the din of the most divisive election campaign here in decades. A full term was not a sure thing when the mercurial Papandreou's Pan-Hellenic Socialist Movement, known as Pasok, swept to power in October 1981.

Although his government has been criticized for strident anti-Americanism, poor economic management and political polarization, Papandreou has established the principle of a democratic alternative to previously dominant centrist and right-wing governments, according to some veteran observers of Greek politics.

"It should have happened long ago," remarked a diplomat otherwise unsparing in his criticism of Papandreou's performance in office, "and was historically necessary."

Papandreou, in this perspective, has added a new dimension to the Greek political establishment and has spurred his adversaries of the center-right to undertake their most important political facelift in generations.

The now 66-year-old premier began by introducing social and political changes that were viewed by many as long overdue by 20th-century European standards. They included abolishing capital punishment and dowry payments, authorizing civil marriage, abortion and equal rights for women, and recognizing the wartime service of leftists, which the previous governments had ignored.

But the list of accomplishments -- besides the building of roads and other infrastructure -- is also viewed by many as surprisingly short for a government with a mandate to carry out change, which was Pasok's 1981 campaign slogan.

"Papandreou wasted a tremendous opportunity," a critic said. "He seems less interested in solving problems than in a dazzling circus act, an exercise in personal power and keeping ambitious rivals off-balance, in which this manipulator of language excels."

In the economic realm in particular, much had been expected of the former Berkeley economics professor. Yet the country's major economic indices have deteriorated in key areas, and Papandreou's policies appear set in a quasi-Marxist mold abandoned by fellow ruling Socialists in France and Spain.

Inflation is down from 25 percent in 1981, but at 18.6 percent is three times the average of other European Community members.

The foreign debt, less than $8 billion at the end of 1981, was nearly $13 billion three years later and is still growing. Domestic and foreign investment has virtually ceased. Unemployment is reckoned conservatively by the Bank of Greece at 8.1 percent -- or more than 300,000 in a population of nearly 10 million.

More would be out of work were it not for the controversial government takeover of money-losing enterprises, a move that the opposition claims has been much more costly than paying laid-off workers unemployment benefits.

The politically popular but economically questionable system of automatically indexing wages for inflation has further complicated efforts to make the archaic economy more efficient and cut the bloated government payroll.

Domestically, Papandreou is on more solid political ground in his stridently anti-American and nonaligned foreign policy, which sets Greece apart from and often infuriates the other members of the North Atlantic alliance and the European Community.

Much of his rhetoric sounds like a throwback to the angry antiwestern statements of such Third World anticolonialists as Ghana's Kwame Nkrumah or India's Krishna Menon a generation ago. But in Greece even many right-wingers and centrists secretly share his criticism of past policies of the United States, often seen as the latest in this century's long line of great powers dictating their will to the Greeks.

Similarly, even his enemies recognize that, for better or worse, Greece is no longer taken for granted, thanks to Papandreou's pronouncements, which range from calling the United States a "metropolis of imperialism" to agreeing with Moscow's claims that the Korean airliner it shot down in 1983 was on a spy mission.

Foreign policy issues have been almost totally absent from the campaign, unlike 1981, when Papandreou touched the deep-seated nationalism in Greeks by threatening to close down U.S. bases, leave NATO and oppose membership in the European Community.

Since last year, Papandreou has toned down his anti-American outbursts. Pasok officials, arguing that his bark is worse than his bite, point to the 1983 agreement extending the leases of four American military installations here. That, they note, was something the previous conservative government did not dare do. And they concede that Greece will not now leave the European Community.

What lies ahead for the U.S. bases is unclear. The Pasok platform demands that the bases, which "expose us to the danger of annihilation" in a nuclear war, be "definitively" removed in accordance with a timetable.

But the 1983 agreement contains no such timetable. Closing the bases would deprive Greece of American military aid, now roughly $500 million annually, and leave its armed forces at the mercy of Turkey, which Papandreou insists is the real threat to Greek security.

More immediately worrisome is the deep cleavage among Greeks that has helped Papandreou to propel Pasok to power, doubling its share of the vote in each of three national elections starting in 1974.

Applying the techniques of North American politics that he picked up during his long stays in the United States and Canada, Papandreou effectively squeezed the center out of Greek politics. In the process, he has reopened many wounds of the Greek body politic dating back to the 1946-49 civil war. He steadfastly refuses to credit the right with restoring democracy after the 1967-74 military dictatorship collapsed and his rhetoric constantly warns of a return to police state techniques if Pasok loses power.