Rep. Choi stopped short of outright criticism of Moon, who is often accused of flexing factional powers, but said the next president must be one who is willing and able to change South Korea's imperious presidency.

"The power that had thoroughly betrayed the people has been brought down. Setting a new absolute power on the very same spot and enjoying the fruits of such power is not right," he said.

Currently, four contenders are vying for the liberal party's nomination in the May 9 presidential election. They include South Chungcheong Province Gov. An Hee-jung, Seongnam City Mayor Lee Jae-myung and Goyang City Mayor Choi Sung.

With Choi's defection, the Democratic Party now controls 120 seats in the 299-seat National Assembly, still the largest number controlled by a single party.

Later speaking to reporters, Choi confirmed his plan to work with former interim chief of the Democratic Party, Kim Chong-in, who defected the party earlier this month.

"I believe Kim will play a pivotal role, and that his role will create a great, successful outcome. And that will make people happy," Choi said of the former party leader.

Choi's defection follows earlier reports that Kim may soon announce his bid for presidency.

Kim earlier vowed to play a "role" in forming an alliance of political parties and presidential hopefuls to field a single, unified candidate who can defeat frontrunner Moon in the upcoming election.

"Preparations for forming a coalition government require efforts to field an unified candidate. A coalition government will be possible only if we successfully complete the unification process. Without that, we cannot lead this country," he said earlier.

Those close to the former lawmaker said Kim may declare his official bid as early as next week. Kim lost his proportional representative parliamentary seat automatically when he quit the Democratic Party.

In apparent efforts to join forces with other presidential hopefuls, Kim has been holding a series of meetings with such figures, including former Prime Minister Chung Un-chan.

Following their one-on-one meeting last week, the two met again earlier Wednesday in a three-way meeting that involved Hong Seok-hyun, another presidential prospect who resigned last week as chairman of major local daily JoongAng Ilbo. (Yonhap)