The Conservatives are more open to Nato membership, but are not pushing the issue amid strong opposition to it in a country that shares a 1,300km border with Russia.
Raimo Vayrynen, director of the Finnish Institute of International Affairs, said party leaders appear to have "some kind of tacit agreement" not to discuss possible Nato membership.
"They have been reading the situation in the manner that you rather lose than win votes if you take up the Nato option," he said.
In the previous election in 2003, the Centre Party narrowly defeated the Social Democrats to take the top spot, and it has maintained its lead.
Finland, home to the world's largest mobile phone maker, Nokia, has a booming economy and an extensive welfare state.
Its main political parties differ little on substance, with broad agreement on foreign and domestic policies.
Some 4.3 million people are eligible to cast ballots in Sunday's vote, which falls on the 100th anniversary of Finland's first elections in 1907, the same year that it gave women the right to vote and to stand for election - the first country in the world to do so.