ATHENS (Reuters) - Greece's Evangelos Venizelos quit as finance minister on Monday to lead the Socialists into a general election, a day after taking over the helm of the party, which is trailing far behind its rival conservatives in opinion polls.

The socialist PASOK party, which came to power in October 2009 on a platform to tax the rich and help the poor but had to change course when the debt crisis exploded weeks later, has been weakened by over two years of unpopular austerity.

Venizelos, 55, who became finance minister in June last year and stayed in the job in a coalition government that took over in November, has spearheaded the marathon debt talks which were concluded with the world's biggest debt restructuring last week.

"We cannot afford to relax or think that we have exited the crisis," Venizelos, 55, told a cabinet meeting, adding that he will fight "for a new parliament which will guarantee stability and a future for the country."

Venizelos, a constitutional law expert turned politician, was elected on Sunday to take over PASOK from former Prime Minister George Papandreou with a mandate to revive the party's chances ahead of the polls.

The name of the new finance minister is expected to be announced on Tuesday, government sources said, adding that Interior Minister Tassos Giannitsis had emerged on Monday as a possible candidate.

Deputy Finance Minister Filippos Sachinidis and the prime minister's chief economic adviser Gikas Hardouvelis were also among the possible choices to replace Venizelos at the finance ministry, source said, while local media speculated that technocrat Prime Minister Lucas Papademos himself may take over as finance minister.

Opinion polls show the election to be held by early May may produce another coalition of the Conservatives and Socialists, who have been warned by the EU and the IMF that Greece must push ahead with more austerity to keep getting the aid it needs to stay afloat.

(Reporting by Lefetris Papadimas and Renee Maltezou; Writing by Ingrid Melander; Editing by Hugh Lawson)