A former liberal lawyer from a humble family, Mr Moon Jae In lost narrowly to conservative political blueblood Park Geun Hye in South Korea's 2012 presidential election.
Now 63, Mr Moon is expected to mount a second bid for the top job in elections next year, but has been consistently outshone by outgoing United Nations chief Ban Ki Moon in opinion polls.
Until now, that is. With Ms Park's approval ratings plunging to a record low in the wake of an influence-peddling scandal involving her longtime confidante Choi Soon Sil, the tide has turned for Mr Moon, who is now the most preferred presidential candidate according to opinion polls.
Pollster Realmeter's survey last week found Mr Moon with an approval rating of 20.9 per cent, compared with 16.5 per cent for Mr Ban, who is said to be considering running for presidency under Ms Park's Saenuri party.
A survey last week by daily newspaper Munhwa Ilbo showed that support for Mr Moon's Democratic Party has surged to 37.5 per cent, as the public turned against Saenuri (26.2 per cent) for condoning the way Ms Park allowed her friend to meddle in state affairs.
While it is still early days, Mr Moon's favourable position in the presidential race may prove extra sweet given that he was arrested and expelled from university for organising a protest against the dictatorial policies of Ms Park's father, the late president Park Chung Hee.
Mr Moon is also a vocal critic of South Korea's first female president, often lambasting her for being "authoritarian and regressive".
Born in Geoje city in South Gyeongsang province in the south-east, Mr Moon grew up in nearby port city Busan and was known to be a brilliant student.
The former student activist passed the bar exam in 1980 - behind bars, for organising anti-government protests.
About two years later, he joined a law firm in Busan, where he worked closely with human rights lawyer Roh Moo Hyun, who later became the country's ninth president.
Mr Roh, a progressive leader who advocated reconciliation with North Korea at the expense of good ties with the United States, roped in his close friend Mr Moon as his key adviser. When Mr Roh committed suicide after becoming embroiled in a graft scandal in 2009, Mr Moon organised his funeral.
After Mr Roh's death, Mr Moon returned to practising law though he was courted by opposition parties .
He returned to politics in the April 2012 general election and won a parliamentary seat for a Busan district. Months later, the Democratic Party candidate ran against Ms Park in the presidential poll.
Last year, he became chairman of the main opposition Democratic Party but stepped down in January, partly to prepare for the next presidential election.
Analysts said the current crisis is a chance for Mr Moon to shine if he can show the leadership needed to take the country forward.
So far, the signs are promising. He was first to propose forming a bipartisan Cabinet led by a neutral prime minister to take over the running of the country from President Park, whose term ends in February 2018.
While Mr Moon has warned that he would have to make a "serious decision" if the political standstill continues, he has refrained from directly demanding Ms Park's resignation, unlike potential rivals Ahn Cheol Soo, founder of the People's Party, and Seoul mayor Park Won Soon. Mr Moon is well aware that this may spark a backlash.