Key government and Socialist Party officials said today that U.S. national interests would not suffer and could even benefit from the elimination of conservative president Constantine Karamanlis from the Greek political scene.

A high-ranking government official, who requested anonymity, said in an interview that the government of Socialist Prime Minister Andreas Papandreou either had settled -- or was close to settling -- all major outstanding bilateral problems with the United States.

He listed various bilateral agreements -- starting with the five-year agreement renewing U.S. military base leases, and including a civil aviation accord. The "last round" of negotiations, he said, covers the status of U.S. forces stationed here and talks on modernizing Voice of America radio installations on which he foresaw "no major difficulty."

He acknowledged possible American concern that Karamanlis is no longer a force balancing the mercurial prime minister, but said key American officials had "insinuated that we are a nasty partner" even before Papandreou refused to back the president's reelection bid.

Citing alleged misleading recent congressional testimony by key U.S. officials, he expressed fears that the United States was tilting in favor of Turkey and seeking to upset the formula guaranteeing Greece 70 percent of whatever American aid that archrival Turkey received.

Meanwhile, a Socialist Party official argued that the elimination of Karamanlis, in fact, served U.S. interest. He said the left wing of the Pan Hellenic Socialist Movement (PASOK), which had become unhappy with the idea of backing Karamanlis for reelection to the point of threatening to vote for the Communists, now would remain firmly in the fold of the prime minister's party.

He warned that any "overt" American action against Papandreou would be counterproductive in the present nationalist mood of Greece and would only ensure the prime minister's victory over his conservative rival Constantine Mistotakis of the New Democracy Party.

Asked what the Reagan administration could do if it wanted to hurt the prime minister's reelection chances, the official said jokingly, "Invite him to the United States on an official visit."

Looking ahead to early legislative elections, which both PASOK and its conservative New Democracy rivals predict will be held in May, the party officials suggested the Communists would lose as much as half their strength. He was confident that the small Eurocommunists would "disappear" as an effective political force and that the larger pro-Moscow party would poll only "5 to 6 percent" -- half their traditional showing.

Left unsaid was the assumption that the Communists had no choice but to vote as Papandreou wanted -- both in the presidential election in parliament starting Friday and in later legislative elections.

With the Communists cut down to a more manageable size -- and the PASOK left wing mollified by Karamanlis' departure -- the party official suggested the prime minister and his party would be serving their own -- and American -- interests.

Despite party insistence that Papandreou still controls the PASOK machinery, the abrupt decision to drop Karamanlis has fueled public questions about the universally accepted notion tha Papandreou alone made all the important decisions.

The party official said demands to oppose Karamanlis' election came not only from left-wingers and the young within the party, but also from older and more moderate members.

However, diplomatic sources and political analysts reconstructing how the decision was made tentatively have concluded that key leftists within the PASOK executive committee carefully orchestrated a last minute campaign of telegrams demanding that Karamanlis be dropped.

Moreover, it is far from clear that the Communists would vote for PASOK presidential candidate Christos Sartzetakis without demanding important concessions. The pro-Moscow party's Central Committee has yet to meet to endorse the PASOK presidential choice and party spokesmen have hinted that they expected a real share of power in exchange for any such endorsement.