Democratic Party presidential candidate Lee Jae-myung registered his candidacy with the state-run election watchdog Saturday for the June 3 presidential election, his officials said.
Former liberal DP leader Lee is leading the polls as the strong front-runner.
He is known for his past hard-line remarks about Japan and support for close ties with China.
In recent months, however, he has expressed strong support for the U.S. alliance and the trilateral partnership with Japan, in an apparent effort to appeal to middle-of-the-road voters.
Attention is being drawn to former Prime Minister Han Duck-soo, who has emerged as a potential game changer for the June election.
Earlier in the day, Han joined the People Power Party to potentially replace Kim Moon-soo as the party's candidate.
