The outcome if confirmed will at the very least leave Yoon as a lame duck for the three remaining years of his term in office, and could even open the way for his impeachment.
The Democratic Party (DP) of Lee Jae-myung — survivor of a January knife attack by a disgruntled voter — and its satellites are forecast to win as many as 197 seats, up from 156 in the last parliament.
Yoon’s People Power Party (PPP) and its partner were projected to be trailing on between 85 to 99 seats, down from 114, the exit polls conducted by three major broadcasters indicated.
All opposition parties combined may even have secured a super-majority of 200 in the 300-seat parliament, which could in theory allow them to attempt to remove Yoon from office before his term ends in 2027.
That includes the new Rebuilding Korea party, led by former justice minister Cho Kuk, which capitalised on discontent with the two main parties to pick up a projected 12-14 seats.
“The people have won, the will to judge the Yoon Suk Yeol administration is very clear,” Cho said after the vote, local media reported.
On the campaign trail, he vowed to make Yoon “first a lame duck, then a dead duck”.
“The figures today show the strong anger of people at Yoon for his two-year governance,” political analyst Yum Seung-yul told AFP.
“What if he won’t change even with this stunning election outcome? I think there will be even more public anger and that worries me.”
Lee’s revenge
Yoon beat Lee in South Korea’s closest-ever presidential election in 2022 and has taken a tough line with the nuclear-armed North while improving ties with Washington and former colonial occupier Japan.
But Lee, while fending off a slew of graft probes he says are politically motivated, has secured revenge with the election result following a bruising and polarising campaign.
“I’ll watch the people’s choice with a humble heart,” Lee said after the vote, local media reported.
From the start of his presidency, Yoon has been unpopular, with ratings hitting the low 30s, and the PPP’s lack of control of the National Assembly stymying his socially conservative legislative agenda.
This includes planned healthcare reforms – that are backed by voters but have sparked a crippling strike by doctors – and a pledge to abolish the ministry of gender equality.
PPP leader Han Dong-hoon said that “exit polls are disappointing… We will watch the vote count”, the Yonhap News Agency reported.
No babies
On Yoon’s side were shifting demographics, with voters aged 60 and older now outnumbering those in both their 20s and 30s in a country with the world’s lowest birth rate.
Younger Koreans have been put off politics by a political class dominated by older men who ignore their concerns.
Many say this was underlined by the horrific 2022 Halloween crowd crush in Seoul that killed more than 150 mostly young people.
The younger generation is also struggling economically, with cut-throat competition in education, fewer job opportunities and sky-high housing costs.
“There is definitely less interest in this election among the people around me than last time. I think it is because they feel rather disappointed,” business owner Kim Yong-ho, 24, said outside a polling station in Seoul’s Gwangjin district.
The tone of the campaigning has also put many voters off, lacking in substantive policy debate and marked instead by shrill calls to “imprison” Lee or “punish” Yoon.
“I am truly ashamed of our country’s politics and government,” Kim Do-kyung, 47, an activist for migrant women and their children, told AFP.
This has been accompanied by hate speech and disinformation online that experts worry could lead to more attacks like the one on Lee in January and another weeks later.
Onions
The DP favours a less hawkish approach towards Pyongyang, and Lee has made a number of pro-China remarks. One doctored video showed him bowing to a statue of Mao Zedong.
It has also latched onto a gaffe by Yoon last month about the “reasonable” cost of green onions, a staple in Korean cooking that has soared in price.
The humble vegetable became a popular prop at DP rallies, and the election commission even banned voters from bringing them to polling stations.
The first official results were expected later Wednesday.