South Koreans have begun voting in a closely-contested parliamentary election seen as a key test for President Lee Myung-bak.
The ruling Saenuri party and the opposition Democratic United Party are neck and neck, analysts say.
While national security issues are a voter concern, domestic economic issues remain the top priority.
This is the first time in 20 years that the parliamentary and presidential polls have fallen in the same year.
The polls for the 300 parliamentary seats occur once every four years. Some 246 legislators will be directly elected by 40 million eligible voters, with the rest selected through proportional representation.
Analysts quoted in South Korean media say that the National Assembly will likely be evenly split between the two rival parties.
Currently the conservative Saenuri (New Frontier Party) has 165 seats in the outgoing parliament, while the liberal opposition DUP has 89 seats.
Early in the race the DUP was tipped to win, with analysts citing Mr Lee's unpopular pro-business policies as a reason.
But opposition mistakes involving its choice of candidates have resulted in a race too close to call, analysts say.
The polls, which opened at 06:00 (21:00 GMT on Tuesday), are seen as a referendum on Mr Lee's term in office and whether his party can retain the presidency in the December election.
One of the key issues dividing the parties is the recently ratified free trade agreement with the US, with the opposition wanting to renegotiate the deal.