Barnaby Phillips, Al Jazeera's correspondent in Athens, said: "The fires have changed the political equation here."
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The socialist PASOK party, led by George Papandreou, the former foreign minister, has accused Karamanlis of failing to protect Greece from the fires and of neglecting promises to combat corruption.
Phillips said: "There is a feeling that the government was slow and the authorities were poorly organised in relation to the fires, but that doesn't only hurt New Democracy ... I think a lot of Greeks also blame the main opposition party, PASOK."
PASOK ruled Greece for most of the 1980s and 1990s, but lost elections to Karamanlis's party in 2004.
But Phillips said that PASOK, was "not necessarily seen as a fresh alternative".
"Instead what you have ... is more fringe parties picking up votes and doing well."
Twenty-one parties are contesting the 2007 elections.
The ruling party has pledged to continue its business-orientated policies, which it says increase competitiveness and spur growth.
Its critics say the party has yet to implement promised structural reforms.
Pledging more funds for education and environmental protection, the socialists say Greece should move closer to the "Scandinavian model" of pro-business policies with a solid welfare state.
Led by Aleka Papariga, the Communists (KKE) are the only party in parliament to be headed by a woman.
The KKE has accused the government of cutting workers' wages, selling off state property and curbing workers' rights.
Known as Syriza, the coalition has a pro-immigrant and strong environmental agenda.
Its popularity dropped in 2004, but it is still expected to achieve the three per cent needed to put it in parliament..
Popular Orthodox Rally
Opinion polls suggest the Popular Orthodox Rally (LAOS), a far-right group, could enter parliament for the first time in this election.
Dionysis Mitrogiannis, a voter on his way to a polling station near the western town of Arta, said: "People here are generally split between the two main parties but many here will vote for smaller parties because they are disappointed."
Greece imposes a blackout on opinion polls ahead of elections, but polls published before September 1 showed Karamanlis's New Democracy leading Papandreou's PASOK party by between one and two percentage points.
Phillips said: "The indications of most experts is that New Democracy will just squeak home.
"They'll probably get around 42 per cent of the vote, which is what they need under the Greek electoral system to get an overall majority ... but they're not over-confident about that."
The two main parties have pledged to create jobs, improve standards of living and shore up Greece's ailing pension system.
Karamanlis has been praised by the European Union for cutting deficits and turning around Greece's economy and he has vowed to push on with the reforms the country needs to catch up with its euro zone partners.
Despite this, Greek per capita gross domestic product is the lowest in the zone next to Portugal's and 20 per cent of the population live below the poverty line.
The far-right Popular Orthodox Rally could achieve the three per cent of the vote needed by a prty to enter parliament.
If this happens, the party could end up acting as a powerbroker.
Papandreou has indicated that he would be willing to form a coalition with other parties but Karamanlis has suggested Greece would be heading for another election if he did not win a strong mandate.
More than 9.8 million people are eligible to vote Sunday, out of a population of 11.4 million, in Greece's 12th parliamentary election since democracy was restored in 1974 following a military dictatorship.
Nearly 500,000 are first-time voters.