Opinion polls indicate that the conservatives and the left-wing opposition parties are in a dead heat in the election that serves as a curtain-raiser for the more important presidential vote in December.
A win for the pro-business, ruling Saenuri party would be a huge boost for its leader Park Geun-hye, the daughter of South Korea's former dictator Park Chung-hee, who has been dubbed the "Queen of Elections" for a string of poll wins in 2003.
Pitted against Park is a left-wing coalition that has vowed to rein in the huge conglomerates that dominate South Korea's economy.
It is pinning its hopes on mobilizing the youth vote that helped deliver it a win in mayoral elections in the capital of Seoul last year.
"The Park Geun-hye effect has been very, very big. She rolled up her sleeves and it has been practically been just her single handed," said independent political commentator Yu Chang-seon said, saying Park stands to gain significantly as long as her party avoids a landslide defeat.
The conservatives currently hold 162 out of 299 seats in parliament and control the presidency and this is the first time in 20 years that both polls have been held in the same year.
Riding on tide of public discontent for the political establishment, human rights lawyer Moon Jae-in has surged into the running for the presidency after playing a role to unify the disparate centre-left into a single coalition late last year.
Moon, who is running for a seat in the southern city of Busan, advocates more welfare spending and closer ties with North Korea, which has defied international warnings to move ahead with a planned long-range rocket launch this week.
The rocket launch and the threat of a third nuclear test by North Korea, which remains technically at war with the rich south, have not been a major factor in the elections.
Despite a rising tide of discontent over growing income inequality, the left, led by the Democratic United Party, has most to lose from a poor showing in the parliamentary polls.
"For the Democratic United Party, it appears to be a case of missed opportunity," said Yu.
The opposition is counting on the power of young voters and social media to boost its chances.
"If Moon Jae-in wins a seat and the Democratic United Party claims a significant number of seats, Moon will emerge as a strong presidential contender," said Lee Jun-han of Incheon University.
Another potential presidential contender who is seen as a possible leader of a united centre-left, software mogul Ahn Cheol-soo, urged voters to turn out to express their discontent with the government.
Ahn, who has superstar status among younger voters, likened the opportunity to cast a vote to hit computer game "Angry Birds".
He put out a clip on Youtube titled "Angry Just Vote" saying younger voters had the chance to turf out the government that backed vested business interests here
He also promised to put on a skirt and dance if turnout reached 70 percent, a level that would virtually ensure the centre-left won as it benefits most from a high turnout.
(Additional reporting by Ju-min Park; Editing by Raju Gopalakrishnan)
Reuters
