South Koreans began voting Wednesday to elect a new four-year parliament in a crucial referendum on President Lee Myung-bak just months before another key poll electing his successor.

Polls opened at 6 a.m., local time, and will close 12 hours later. A total of 246 lawmakers will be directly elected by some 40 million eligible voters, while the other 54 seats will be allocated based on proportional representation.

A clear picture of the general elections has not emerged, with surveys showing the president's ruling Saenuri Party and the main opposition Democratic United Party neck and neck.

The Saenuri Party currently holds a comfortable parliamentary majority, but a series of scandals involving officials in the Lee administration hit hard the conservative party that went on to change its name to distance itself from the beleaguered president.

The party itself has made a series of decisions unpopular among the young. It led the railroading of a controversial trade agreement with the United States, and is pressing ahead with the construction of a naval base in a southern resort island despite mounting criticism.

Most recently, allegations surfaced government officials illegally spied on civilians, a claim that shook the ruling camp just weeks before the general elections.

Mishaps of the conservatives proved to be a boon for the liberals, who managed to form broad alliances now represented by the Democratic United Party and the left-leaning minor opposition Unified Progressive Party.

It was a short-lived boon, however, as the liberals struggled with infighting and failed to extinguish a controversy involving one of its candidate whose lewd remarks from the past came back to haunt him.

Experts say voter turnout is a critical factor deciding poll results, which is expected to be announced early Wednesday morning.

Parties see 135 to 140 seats as a minimum threshold to win a majority of the seats.