South Korea's ruling party suffered a humiliating defeat in Wednesday's by-elections, heralding a political upheaval ahead of next year's presidential polls.
In the most closely-watched contest, Sohn Hak-kyu, leader of the main opposition Democratic Party (DP), defeated Kang Jae-sup, former head of the ruling Grand National Party (GNP).
Sohn's win bolstered his profile as a potential presidential candidate.
Choi Moon-soon of the DP was also elected as governor of Gangwon Province over Ohm Ki-young of the GNP, brightening the DP's prospects for the general elections next year.
In Gimhae, South Gyeongsang Province, Kim Tae-ho, formerly governor of the province, made a successful return to politics. He was nominated as prime minister in August last year but he gave up the nomination amid alleged ethical lapses revealed during confirmation hearings.
Political watchers said the GNP's defeat added burden to President Lee Myung-bak haunted by the specter of an early lame duck. The GNP is expected to come under growing pressure to change its leadership, they added.