ATHENS – Greece’s Socialists trounced the governing conservatives in a landslide election Sunday, with voters angered by scandals and a faltering economy ousting Prime Minister Costas Karamanlis halfway through his second term.

Humbled by his New Democracy party’s worst electoral performance ever, Karamanlis, 53, resigned as its leader and said a new chief is needed for the party founded by his late uncle Constantine Karamanlis 35 years ago.

George Papandreou, 57, now follows in the footsteps of his father, Andreas Papandreou – who founded his Panhellenic Socialist Movement party, or PASOK – and grandfather and namesake George Papandreou, both of whom served several terms as prime ministers.

“We bear a great responsibility to change the course of the country. … We know that we can make it,” Papandreou, a former foreign minister, told jubilant supporters lighting flares and waving PASOK flags depicting the party’s symbol of a green rising sun outside his party headquarters in central Athens.

“I know the potential of the country very well, a potential being drowned by corruption, favoritism, lawlessness and waste,” Papandreou said. “We must all believe again we can succeed … we cannot waste a single day.”

Results from 82.07 percent of votes counted showed PASOK winning with 43.92 percent, compared to 34.15 percent for New Democracy. Turnout was at 70.24 percent. Voting is compulsory in Greece, although penalties for failing to vote are no longer enforced.

The result gives PASOK a comfortable majority of 159 seats in the 300-member parliament, bringing the party back to power after five years of conservative governance.

Papandreou’s victory, along with a recent election win by socialists in Portugal, bucks a European trend that has seen a conservative surge in the continent’s powerhouse economies, including most recently in Germany, where Chancellor Angela Merkel won re-election last week.

“This is a historic victory for PASOK, which means great responsibility for us,” said senior party official and former minister Evangelos Venizelos.

Papandreou will now have to deal with a faltering economy that is expected to contract in 2009 after years of strong growth, while the budget deficit will probably exceed 6 percent of economic output.

In contrast to Karamanlis, who advocated an austerity program of freezing state salaries, pensions and hiring, Papandreou has promised to inject up to $4.4 billion to jump-start the economy.

However, his government will likely have to borrow heavily just to service the ballooning debt – set to exceed 100 percent of GDP this year – and keep paying public sector wages and pensions. Papandreou has pledged to limit borrowing by reducing government waste and going after tax dodgers.

Thousands of cheering supporters mobbed a smiling Papandreou as he arrived at the central Athens headquarters while the results trickled in. Others drove through the city honking their horns.

Karamanlis, looking tired and downcast, congratulated his rival.

“From the depths of my heart, I wish to thank the voters who backed us in these elections. I wish to congratulate George Papandreou for his victory,” he said in a brief speech in central Athens. “We hope he succeeds in the great challenge of facing the economic situation.”