"I have nothing more to do in the party," Kim said through his social media account. "I will have some time to contemplate on what to do while watching the political situation after the ruling on the impeachment."

President Park Geun-hye was impeached by parliament in December. The Constitutional Court is widely expected to deliver its final ruling as early as this week.

Kim, a proportional representative lawmaker, automatically lost his parliamentary seat with the defection and it will be handed over to Shim Ki-joon, a member of the party's Supreme Council.

Pundits said the departure came in an apparent bid to form an alliance of centrists to challenge front-runner Moon Jae-in in the upcoming presidential election.

Moon invited Kim to become interim leader in January last year in line with the party's preparations for the general election, which took place in April. Kim is considered a figure who holds connections with both the left and right. (Yonhap)