The socialists hope to claw back support at the last
Campaigning has ended ahead of Greece's general election on Sunday, which the conservative opposition is tipped to win by a narrow margin.
The governing socialists held a massive rally in Athens in an appeal to voters for another chance after ruling almost continuously since 1981.
Opinion polls have been giving Costas Karamanlis' conservatives a lead of only three points.
But the large number of undecided voters means an upset is possible.
Both Mr Karamanlis and the socialist Prime Minister, George Papandreou, come from families that have dominated politics since the 1950s.
The election comes just five months before the Athens Olympics but Mr Karamanlis' New Democracy Party has promised not to make changes to the team charged with the huge effort for the games.
"We will not replace any of the key people organising the Olympics after the elections," party spokesman Theodoros Roussopoulos told the Associated Press news agency.
Mr Papandreou has accused the conservatives of being too inexperienced to either run the games or handle the delicate peace talks on Cyprus ahead of EU accession in May.
Tens of thousands of supporters of the governing Pasok party turned out in the centre of the capital and Mr Papandreou went on television to say the party had made mistakes but deserved to stay in office.
The conservative lead has ebbed away in the campaign's last weeks
"We recognise that weaknesses exist but there is the will to change those badly written pages," he said.
Referring to corruption, he said a new Pasok government would show "zero tolerance for shady dealings".
The prime minister also announced lavish, last-minute spending promises.
Mr Karamanlis, who took over the leadership of the New Democratic Party seven years ago, has promised to eliminate corruption, streamline bureaucracy and tackle an unemployment rate of nearly 9%.
He is the nephew of former prime minister and party founder Constantine Karamanlis.
The BBC's Richard Galpin in Athens says there is no doubt the conservatives have the best chance for more than a decade of unseating the socialists.
But things could change, he says, with their lead in the opinion polls narrowing recently after months when they appeared set to return to power.