SEOUL South Korea's liberal opposition parties scored a landslide victory in a parliamentary election held on Wednesday, dealing a resounding blow to President Yoon Suk Yeol and his conservative party but likely falling just short of a super majority.

The Democratic Party (DP) was projected to take more than 170 of the 300 seats in the new legislature, data by the National Election Commission and network broadcasters showed with more than 99% of the votes counted as of 5.55am on Thursday (2055 GMT Wednesday).

A splinter liberal party considered allied with the DP was expected to take at least 10 seats, projections showed.

"When voters chose me, it was your judgment against the Yoon Suk Yeol administration and you are giving the Democratic Party the duty to take responsibility for the livelihood of the people and create a better society," DP leader Lee Jae-myung said.

Lee won a seat in the city of Incheon to the west of the capital, Seoul, against a conservative heavyweight candidate considered a major ally of the president.

The bitterly fought race was seen by some analysts as a referendum on Yoon, whose popularity has suffered amid a cost-of-living crisis and a spate of political scandals.

"Judgment" was the common theme running through comments by opposition victors, many of whom had campaigned heavily focused on what they said was Yoon's mismanagement of the economy and his refusal to acknowledge his wife acted improperly when she accepted a Dior bag as gift.

First lady Kim Keon Hee has not been seen in public since Dec 15 and was absent when Yoon voted, reflecting the view by some analysts and opposition party members that she had become a serious political liability for the president and his PPP.

His People Power Party (PPP) was projected to win just over 100 seats, meaning Yoon would avoid the super-majority of a two-third opposition control that could break presidential vetoes and pass constitutional amendments.

But nearing the end of the first two years of his five-year single term allowed by the constitution, Yoon was likely to slip into a lame duck status, some analysts said.

The National Election Commission (NEC) was expected to announce the official results later on Thursday. Nearly 29.7 million people, or 67% of eligible voters, cast their ballots, according to the NEC.

It marked the highest ever turnout for a parliamentary election, though the numbers were down from the 2022 presidential vote that narrowly brought Yoon to power.

Yoon, who took office in May 2022, was not up for election this time but his ability to pass legislation is likely to be badly damaged by the poor showing by his PPP.

He has suffered low ratings for months, hamstrung in implementing his pledges to cut taxes, ease business regulations and expand family support in the world's fastest ageing society.

Mason Richey, a professor at Hankuk University of Foreign Studies, said Yoon might focus more on his overseas agenda now, though those plans could also be at risk if the opposition seeks to cut budgets with its majority.

"Given his likely lame duck status, the temptation for Yoon will be to focus on foreign policy where he will still have statutory power," Richey said.

  - Reuters