Opposition leader Constantine Mitsotakis, describing socialist Prime Minister Andreas Papandreou's dramatic realignment of presidential politics here Saturday as a blunder, is insisting that power is now within reach of the conservative New Democracy party.

In a startling about-face, seemingly designed to mollify vocal left-wing radicals within the Pan Hellenic Socialist Movement (Pasok), Papandreou abruptly withdrew his promised support for conservative President Constantine Karamanlis' reelection bid.

For Mitsotakis, the departure of Karamanlis -- who was the founder of New Democracy -- is a mixed blessing. The 78-year-old president's presence was perceived as a safety net preventing the mercurial prime minister from carrying out threats to leave NATO, close down U.S. military bases or pull out of the European Community.

Yet centrists, who traditionally decide Greek elections and who voted massively for Pasok in 1981 because of Karamanlis' presence, are faced with the prospect of an out-and-out left-wing rule now that he has gone. Centrist second thoughts have also increased because of Papandreou's proposed constitutional amendments drastically curtailing presidential powers.

The voters could make their choice as soon as May 5. Papandreou, in a speech to party workers following his shift on Karamanlis, indicated he would call elections for that date although they are not required until October.

Papandreou's move against Karamanlis has triggered opposition charges of deceit, sly practice and unreliability -- the same charges that Mitsotakis has had leveled against himself.

Mitsotakis, 66, who took over leadership of New Democracy last October, hopes to cancel out the black mark he has carried for 20 years and win back centrists.

Many Greeks still agree with Papandreou's charges, in 1965, that Mitsotakis was a "traitor" and "nightmare." The prime minister used those words to describe his rival's role at that time in helping bring down the centrist government of George Papandreou, Andreas' father, by defecting from party ranks and taking part in an unsuccessful right-wing Cabinet favored by the royal family.

Andreas Papandreou and others charge that Mitsotakis' move was a powerful contributing factor to the Army coup that imposed a military dictatorship from 1967 to 1974.

But in an interview in his gloomy parliamentary office, Mitsotakis said he would be pressing charges of unreliability against Papandreou and "I'll be saying it very clearly and every day. Andreas is on the defensive."

Arguing that he has the "vast majority" of Greeks behind his pro-western and free-enterprise policies, Mitsotakis said the "totally unacceptable hoodwinking of Karamanlis" now "makes it much easier" to beat Papandreou. Other political analysts say Mitsotakis faces a tough, uphill battle against the charismatic prime minister, who is a masterful tactician.

Mitsotakis clearly intends to disturb centrist consciences by hammering home the de facto alliance between the Communists and Papandreou, whom he described as a "Marxist and an extremist" who "no longer wants Greece to be a part of western society."

Noting that the Communists hold most of the supplementary votes needed to elect Pasok's presidential candidate, Supreme Court Judge Christos Sartzetakis, in the parliament this month, Mitsotakis said, "This support shows it is no longer sure Papandreou wants to stay in the West."

While placing himself four-square in the western camp, and saying "80 to 85 percent of the Greeks" are against confrontation with the United States, Mitsotakis acknowledged that "certainly there are objective reasons to criticize Washington."

With Papandreou tapping a deep nationalist current of anti-Americanism -- in large part due to Washington's backing of the military dictatorship -- Mitsotakis also indicated concern about any overt support or criticism of his adversary by the Reagan administration.

"The U.S. government should be very careful," he said, adding nonetheless that "our people do not want to be deprived of American friendship, support and assistance."