Kim Dong-yeon, the presidential candidate of the minor New Wave Party, dropped out of the race Wednesday to support ruling party candidate Lee Jae-myung.
Kim, who served as President Moon Jae-in's first finance minister, announced the withdrawal a day after he reached an agreement with Lee of the ruling Democratic Party to have policy coalitions and operation of a joint government.
"It was not an easy decision for me," Kim said. "Please understand this was a difficult decision aimed at realizing the philosophy and values that we pursue."
The two earlier promised to pursue various political reform measures together, including a revision of the constitution and electoral system.
Kim's withdrawal came as Lee is running neck and neck with Yoon Suk-yeol of the main opposition People Power Party (PPP) at around 40 percent support each in opinion polls.
Recent surveys have shown that Kim has less than 1 percent support. (Yonhap)