SEOUL, South Korea -- The millionaire software mogul who was widely seen as a top contender for South Korea's presidential election in December unexpectedly withdrew his candidacy on Friday, throwing his support behind the candidate of the main opposition party who has championed aggressive engagement with North Korea and "balanced diplomacy" with Washington and Beijing. South Korea's diplomatic policy is currently weighted more toward the United States.
The surprising move by the software mogul, Ahn Cheol-soo, has turned the campaign for the Dec. 19 election into a close two-way race between Moon Jae-in, the candidate of the liberal opposition Democratic United Party, and Park Geun-hye, the nominee of the governing conservative Saenuri Party.
Recent surveys predicted that Mr. Ahn, if he became the single candidate representing the liberal South Koreans, would beat Ms. Park, while Mr. Moon would be a much weaker rival against her. But in the same surveys, South Koreans indicated that if Mr. Ahn and Mr. Moon joined forces, voters preferred Mr. Moon as the unified liberal candidate, apparently because of Mr. Ahn's lack of government experience.
The two, who share liberal goals like the narrowing of income inequality and reform of the country's family-controlled and scandal-ridden conglomerates, believed that if both of them ran and split the liberal vote, Ms. Park, their common enemy, would surely win. Ms. Park is the daughter of Park Chung-hee, who established a military dictatorship and ruled South Korea for nearly two decades until his assassination in 1979.
Mr. Ahn and Mr. Moon had been negotiating over who should step aside. Their talks stalled with less than a month left before the election.
But on Friday, Mr. Ahn, 50, an independent, said that he had decided to sacrifice his presidential ambition for the sake of removing the conservatives from power. President Lee Myung-bak, a member of the conservative party, by law cannot seek re-election.
"Now, I will fight as an ordinary foot soldier for a change of governments," Mr. Ahn said at a news conference as many of his volunteers sobbed. "Although I am putting aside my political dream, I really long for a new era and new politics."
Mr. Ahn, a former physician and university dean, recently became a political star. But critics have voiced concern about his lack of experience in government and politics.
In South Korea, such a gesture of sacrifice by a major candidate like Mr. Ahn is unprecedented. In the presidential race in 1987, two opposition candidates, Kim Young-sam and Kim Dae-jung, who hated each other more than their common enemy -- the military dictatorship -- both ran, splitting the vote and helping Roh Tae-woo, the military-backed candidate, win.
"We all owe a huge debt to Mr. Ahn," Mr. Moon was quoted as saying by a party spokesman. "I feel both sorry and grateful to him."
Although recent polls showed that Mr. Moon would lose to Ms. Park in a two-way race, Mr. Ahn's endorsement was widely expected to increase his chances.
Mr. Moon, 59, a former human rights lawyer, was a close ally of former President Roh Moo-hyun, who stepped down in 2008 and committed suicide the next year. Mr. Moon had served in various political posts under Mr. Roh, including chief of staff.
As a student activist in the 1970s, Mr. Moon was jailed for opposing the dictatorship of Ms. Park's father.
He has vowed to reverse President Lee's policy of linking economic assistance to progress in talks on ending North Korea's nuclear weapons program. Like other liberals, he sees economic cooperation essential to reconciliation and eventual inter-Korean reunification. He is eager to hold a summit meeting with the North's new leader, Kim Jong-un.
South Korea cannot postpone "balanced diplomacy" between the region's two biggest powers anymore, Mr. Moon said in a speech this month. "Our alliance with the United States is being steadily maintained and developing maturely," he said, adding, "The economic relationship between China and us is rapidly growing."
This article originally appeared in The New York Times.