The triumph in the first elections since President Moon Jae-in took office around a year ago is widely seen as a boon to the country's leader, who helped bring together the Cold War foes for an unprecedented meeting.
Exit polls by three major broadcasters suggested the DP won 14 of the 17 metropolitan mayoral and gubernatorial posts contested in today's vote.
These included the capital Seoul, whose current DP mayor Park Won-soon was predicted to retain his post for a third term -- cementing his status as a top presidential candidate when Moon steps down in 2022.
The DP was also forecast to secure 10 of the 12 National Assembly seats up for grabs in parliamentary by-elections held simultaneously, with the remaining two seats going to the main opposition Liberty Korea Party (LKP) and an independent.
This will bring the number of DP seats to 128 in the 300-seat parliament, followed by LKP's 114.
The other seats are shared by five splinter parties and five independents.
The LKP secured only two posts, winning the mayorship in its powerbase Daegu City and the governorship of the nearby province of North Gyeongsang, according to the exit polls.
Moon, who has prioritised seeking rapprochement with the North, played a key role in arranging the summit between US President Trump and the North's leader Kim Jong Un in Singapore this week, which focused on dismantling the North's nuclear arsenal.
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