A coalition government
Without an absolute majority, the party that wins the most votes -- only Syriza or the conservative New Democracy party are in a position to do so -- will have to forge alliances.
The third party, the neo-Nazi Golden Dawn, is not expected to win more than seven percent, trailed by several other smaller parties.
The vote leader could seek alliances with Potami and Pasok, small parties that both voted for the new bailout in August.
The centrist Potami, formed in 2014 by prominent TV journalist Stavros Theodorakis, previously had 17 seats in parliament.
The once all-powerful socialist Pasok party, which also had 17 seats, in July elected its first woman leader, Fofi Gennimata, who has called for unity to take Greece forward.
One-party government for Syriza
After winning 36.4 percent of the vote in January elections, the party of former premier Alexis Tsipras narrowly failed to secure an absolute majority in parliament.
With 149 out of 300 seats, he formed a coalition with the small nationalist ANEL party to add 13 MPs to his voting bloc.
Tsipras, who resigned on August 20, is now seeking re-election on pledges to negotiate softer terms with Greece`s international creditors.
The 41-year-old former premier is struggling to recover from his decision in July to accept a third EU bailout worth 86 billion euros ($95 billion) to ward off a debt default and forestall a calamitous exit from the eurozone.
In the space of a few weeks, nearly a fifth of his lawmakers split away to form Popular Unity, a new anti-austerity party of eurosceptics.
To the right, Tsipras was also surprised by New Democracy, which is gaining in popularity thanks to the efforts by its new leader, Evangelos Meimarakis, to move it closer to the centre.
After a poll last Friday showed Syriza five points ahead of New Democracy, a slew of new surveys on Sunday showed them running neck and neck.
Off the record, a Syriza official said the party was banking on a score of 34 percent, down 2.4 points from January, in the September 20 polls.
If a large chunk of the 15 percent "undecided" -- who analysts say are mainly former Syriza voters -- refuse to give the party a second chance, a majority for the anti-austerity party will be out of reach.
Unity government: New Democracy`s ambition
Meimarakis insists that Greece needs a government of national unity after years of economic and political crisis.
"I firmly believe that the new government should be built with as many parties as possible. If the Greek people choose New Democracy to prevail, as it seems will be the case, I will try to achieve really broad cooperation of the political parties of the European front. I will certainly include Syriza," he said on Saturday.
So far, however, Tsipras has vehemently ruled out cooperating with New Democracy.
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