The ruling Saenuri Party won a landslide victory in Wednesday's parliamentary by-elections, beating the main opposition New Politics Alliance for Democracy in its traditional stronghold of South Jeolla Province.
Shocked by the defeat, the NPAD has descended into chaos as co-leaders Ahn Cheol-soo and Kim Han-gil offered to resign.
The by-election was seen as a mid-term verdict on President Park Geun-hye. One of her close aides, Lee Jung-hyun, made history by becoming the first conservative candidate to win a parliamentary seat in the constituency of Suncheon and Gokseong in South Jeolla Province, a traditional stronghold of the opposition.
The ruling party swept 11 out of 15 National Assembly seats up for grabs, which will boost the number its seats to 158 in the 300-seat National Assembly, giving it a comfortable majority.
Saenuri campaigned on economic revival, while the NPAD called on voters to deliver their verdict on the government's mishandling of the April 16 ferry disaster and its aftermath.
Smarting from declining public support due to the ferry disaster, the Saenuri Party had hoped to win five or six constituencies at most but managed to pull off a landslide.
In the Dongjak district in Seoul, the most intensely contested seat, Saenuri candidate Na Kyung-won as expected beat Roh Hoe-chan of the Justice Party, although Roh was the sole opposition candidate after joining hands with the NPAD at the last minute.
No more major elections lie ahead until the general election in April 2016.
The NPAD's defeat shatters an alliance forged only in March with the merger of the former Democratic Party and Ahn's independent camp.
Ahn, a software tycoon, initially enjoyed massive support by promising to reform traditional politics but disappointed many when he merged his outfit with the established opposition.