SEOUL A South Korean lawyer jailed for protesting against military rule nearly 40 years ago, and who advocates closer ties with rival North Korea and more welfare spending, has surged into the running for the presidency later this year.
Moon Jae-in, 59, has emerged as the kingmaker in the resurgent opposition after playing a catalytic role in unifying the disparate liberal groups into one party late last year, analysts say.
Opinion polls show Moon's popularity has risen sharply this year. A Realmeter survey this week indicated he would pip conservative favourite Park Geun-hye in the December presidential vote.
South Korea will vote for a new president when incumbent Lee Myung-bak's mandatory single five-year term ends. Analysts say the vote is wide open, and the main issues will be the economy, job creation, and the growing wealth divide.
The liberal opposition is also headed for a big victory in an April parliamentary vote, polls show, potentially ending conservative rule. Such a result could impinge on Seoul's close ties with Washington.
Moon is the third person in the past six months to emerge as a potential leader of Asia's fourth biggest economy.
Software magnate turned university professor Ahn Cheol-soo opened up a 10-percentage-point winning margin over Park late last year. Ahn had been seen as the best liberal hope of winning the presidency, but polls show his support falling as he repeatedly plays down his political ambitions.
Conservative Park had been favourite for three years until the middle of 2011.
While Moon has not said whether he will run, he has technically left open the door open by turning down a leadership role in the united opposition party.
He told Reuters in an interview his top priority was helping the opposition steal the conservatives' majority in the April parliamentary election. He has declared he will contest the conservative stronghold of Busan.
"I will think about the presidential elections when the time comes," said Moon, who was chief of staff to late President Roh Moo-hyun and an architect of South Korea's "Sunshine Policy" of engagement with and generous aid to North Korea.
Park, the daughter of the country's former military ruler Park Chung-hee, has also not declared whether she will run, but is considered a near certainty.
Moon has raised his profile in the media this year and has managed to capitalise on the conservatives' decline in popularity amid a series of corruption scandals.
Woo Jung-yeop, of the Asan Institute for Policy Studies, said Moon had managed to portray himself as a moderate and rational leader who has the backing of the younger generation.
He said that if Moon decided to run, Ahn would likely step aside to back him, boosting his chances of victory.
From his office in Busan, former human rights lawyer Moon criticised Park as "very authoritarian and regressive". Moon was jailed in 1975 for joining street protests against the dictatorial rule of Park's father.
"There are clear directions where our nation needs to go: we need to strive for more democracy, more welfare, more peace in North and South Korean relations, more interest in anti-pollution, and more improvement in labour conditions," he said.
While North Korea is not expected to figure prominently in South Korea's elections this year, an Asan Institute survey published last month showed that respondents viewed the opposition as being more capable of handling inter-Korean ties.
A Congressional Research Service report prepared for members and committees of the U.S. Congress in January said U.S.-South Korean relations would be more difficult to manage if the liberals regain power.
The United States has some 28,000 troops in South Korea, and Washington is warily watching to see how a transition to a new leadership is progressing in North Korea following the death of iron-fisted leader Kim Jong-il in December 2011.
(Additional reporting by Iktae Park; Editing by Jack Kim and Paul Tait)