Victorious Conservatives Promise Reforms in Greece : Elections: Mitsotakis is assured of a majority in Parliament. The ailing economy is his top priority.

ATHENS —  Greek conservatives returned triumphantly to political power Monday, promising broad economic reforms to roll back eight years of combative socialism and to repair strained relations with American and European allies.

The Athens stock market jumped nearly 15% to record highs Monday when it became clear that conservative leader Constantine Mitsotakis would have a slender but emphatic absolute majority in the 300-seat Parliament.

“It’s an entirely new era,” said one stock trader, reflecting private-sector confidence that a Mitsotakis government will move swiftly to invigorate Greece’s ramshackle economy and recover the confidence of its wealthy partners in the European Community.

In Sunday’s election, the third in 10 months, Mitsotakis’ right-center New Democracy Party won 150 seats on 46.9% of the vote, final official returns showed Monday.

Assurance that Mitsotakis, 71, would have a majority that eluded him in elections last June and November came at midday Monday when Constantine Stephanopoulos, a prominent lawyer, pledged that his centrist Democratic Renewal Party’s lone parliamentary deputy would vote with New Democracy.

In a nation where right-left passions are deep and finely etched, Sunday’s defeat may prove the last hurrah for wasp-tongued Socialist Andreas Papandreou, prime minister from 1981 until financial scandals cost him a majority in the June elections.

Greece has been governed by powerless interim governments since then, while Papandreou, a 71-year-old former Berkeley economics professor who faces trial on corruption charges, worked patiently on a comeback.

Instead, the man who dominated Greek politics for a decade emerged from Sunday’s vote as a battered also-ran. Papandreou’s Panhellenic Socialist Movement, or Pasok, won 38.6% of the vote and 123 seats. A Communist-led coalition with which Papandreou had hoped to form a governing majority got 10.3% and 19 seats.

Of the remaining eight deputies elected Sunday, only the Stephanopoulos supporter has common ground with New Democracy. The other seven include four independent leftists pledged to the Socialists and Communists, an aloof Ecologist Party supporter and two leftist representatives of Muslim communities in northern Greece.

Mitsotakis, a stolid, dogged professional politician, said Monday that he hopes to form a government by Wednesday.

On the political side of a new government’s agenda will be early election of a new figurehead president for Greece. President Christos Sartzetakis’ term is up, but deadlock in the interim parliaments since June failed to produce a successor.

Mitsotakis is also expected to seek electoral reforms that would allow New Democracy to improve its parliamentary majority. He charges that Papandreou-engineered reforms stressing proportional representation cost New Democracy the June and November elections. New Democracy supporters tirelessly note that, with 42% of the vote in her last reelection, Prime Minister Margaret Thatcher won 375 of 650 seats in the British Parliament.

Mitsotakis has also pledged prompt upgrading of diplomatic relations with Israel to full ambassadorial level. Papandreou had balked at that in deference to Arab sensibilities.

For the United States, a majority conservative government should mean quickly resumed negotiations on renewing leases for American military bases here, an issue that was a standby target for the Socialist and Communist campaigners.

In January, the United States announced that it would close controversial Hellenikon Air Base at Athens airport and a communications center at nearby Nea Makri, pleasing some Greeks but alarming others worried at the loss of local employment at the bases and the implications for American military assistance that will total about $350 million this year.

In resumed talks, a Mitsotakis government is expected to agree to new leases for two bases on Crete and about 20 smaller facilities.

Potentially more incendiary for Mitsotakis in a country that is no stranger to terrorist violence is the American demand for jailed Palestinian Mohammed Rashid, who is wanted in the United States for the in-flight bombing of an American jetliner over Hawaii in 1982. Interim governments have lacked the political strength to honor an American deportation request approved by Greek courts.

Greek analysts believe that Mitsotakis will approve the deportation, but they expect him to tread lightly.

ELECTION RESULTS: 1989-90 It took Greeks three elections over 10 months to elect a government in which a single party was assured of majority control of the 300-member Parliament. Here is how the voting went:

POPULAR VOTE % / SEATS

New Democracy Pasok Communists Others June 89 44.2/145 39.1/125 13.1/28 3.5/2 Nov. 89 46.2/148 40.7/128 11.0/21 2.1/3 April 90 46.9/150 38.6/123 10.3/19 4.2/8*

* The Democratic Renewal Party said its single deputy will vote with New Democracy, giving it a majority.