Buoyed by his reelection victory, Socialist Prime Minister Andreas Papandreou is thought likely in the short run to soft-pedal his antiwestern rhetoric and threats to close down U.S. bases in Greece.

This is the view of diplomats and political analysts who said today that his convincing defeat of the Communists, who lost ground from their 1981 electoral showing and did not achieve their hoped-for balance of power position in a hung Parliament, relieves immediate pressure on him to follow up on earlier radical foreign policy promises.

Papandreou's showing against the Communists was regarded as the key to his triumph over Constantine Mitsotakis' center-right New Democracy party, which had hoped that the Communists would get enough votes to prevent the Socialists from forming a government.

Papandreou's advisers privately concede that the faltering Greek economy must be his top priority if the second term of his left-wing government is to succeed.

During the campaign, Papandreou played down the foreign policy themes that proved so successful in his 1981 triumph -- closing the U.S. bases, leaving the North Atlantic Treaty Organization and opposing Greek membership in the European Community.

In Washington, White House spokesman Larry Speakes said President Reagan was sending congratulations to Papandreou and expressed hope for improved relations.

Had the pro-Moscow Communist Party succeeded in holding the balance of power, Papandreou might have had to entertain a high foreign policy price to enlist their support in forming a new government with his Panhellenic Socialist Movement (Pasok).

The Communists had made clear that their asking price included moving toward the definitive closing of the four major U.S. bases and other installations as well as leaving NATO and the community.

On the stump, Papandreou never went beyond his party's vaguely worded platform plank, which said that the bases would be closed during a second term of office "according to a timetable."

But the open-ended agreement on the bases signed in 1983 contains no such timetable and his victory essentially leaves the prime minister even freer to maneuver, according to political analysts.

While the possibility of Papandreou returning to antiwestern threats if the political climate seems to require it cannot be ruled out, analysts said today, he is, for the moment, riding high.

Final returns today showed Pasok with a commanding 45.8 percent of the vote and 161 of the 300 parliamentary seats, followed by New Democracy with 40.8 percent and 125 seats, the pro-Moscow Communists with 9.9 percent and 13 seats and the Eurocommunists with 1.8 percent and a single representative. Minority parties that did not win seats had the remaining votes.

The fall in the Communist vote -- down a full percentage point from 1981 -- also dashed New Democracy's hopes of either winning a parliamentary majority or of becoming the front-runner and forming a temporary minority Cabinet before calling new elections.

Mitsotakis had hoped to win back centrist votes after Papandreou withdrew support in March from the reelection bid of conservative president Constantine Karamanlis, who had been considered a balance against the prime minister's more radical tendencies.

Calculating that the Communists and New Democracy would chip away at his electorate from either end unless he dropped Karamanlis, Papandreou gambled and won.

With the failure of Mitsotakis' opening of the traditionally right-wing New Democracy to the center, his continued stewardship of the party is now in question.

By the admission of his own lieutenant, Mitsotakis' issues-oriented campaign concentrating on lessening the traditionally large state role in the economy, encouraging market forces and attracting private investment failed to get through to enough voters.

The frustrating of both the Communists' and New Democracy's separate strategies served to avoid what diplomats had feared could have been a period of instability followed inevitably by fresh elections.

It was unclear whether, as Papandreou claimed, his reelection represented a mandate to continue the socialist transformation that has become his trademark, or a reluctance of the majority to return to conservative politics that dominated this country before 1981.

Papandreou's great success in co-opting the once-flourishing center in Greek politics may yet come a cropper unless he deals effectively with the economic situation, according to political analysts.

Inflation is running at 18.1 percent, three times the European Community average. Private investment is at a virtual standstill. Unemployment is conservatively estimated at 8.1 percent of the work force and is increasing. The foreign debt has grown from less than $8 billion in 1981 to more than $13 billion and is still rising.

So far Papandreou, a former economics professor, has favored a modified welfare state, extending pensions and insurance benefits for workers while pumping state money into money-losing enterprises to prevent further unemployment.

Whether Papandreou will follow fellow Socialists such as French President Francois Mitterrand and Spanish Prime Minister Felipe Gonzalez in applying austerity measures is the most pressing question facing the new government he is now putting together.

Harilaos Florakis, leader of the pro-Moscow Communists, has charged that Papandreou's second term will be "more conservative" and warned that "the postelectoral class struggle will be intensified."