Moon Jae-in (pictured above, top), a former chief of staff to president Roh Moo-hyun, won the presidential nomination on Sunday of the main opposition party in South Korea, the Democratic United Party (민주통합당, or the ‘Minju Tonghap-dang’), and today, Ahn Cheol-soo (pictured above, below), a popular doctor-turned-entrepreneur launched an independent bid for the presidency.

Both are, somewhat, novices to electoral politics in South Korea.  Moon is the former chief of staff to the late former president, Roh Moo-hyun, and Ahn is a complete outsider to South Korean politics.

Both pull support from the same pool of generally liberal and moderate voters — meaning that if neither drops out before the December 19 election, neither currently would stand much of a chance against the frontrunner, Park Geun-hye of the Saenuri Party (새누리당 or the ‘Saenuri-dang’ / New Frontier Party).

The essential fact of the South Korean presidential race is that if neither Moon nor Ahn steps down in favor of the other, Park will win the election.  Every poll, including the latest one from Real Meter, conducted Sept. 17-18, demonstrates this glaring threshold truth: Park leads with around 39%, Moon follows with 26% and Ahn wins 22.5%.  Although over the course of the month, Moon has moved from 15% and third place (drawing support from both Park and Ahn) into a stronger second place, arithmetic is arithmetic.  Moon and Ahn may seesaw as the favorite challenger to Park, but Moon, having easily won DUP’s primary, now faces another sort of primary — with Ahn — to consolidate the liberal and moderate vote.

Furthermore, even if one candidate bows out soon enough to allow for a unified challenge to Park, the road that either Moon or Ahn faces ahead is tricky.

Even in a one-on-one race with Park remains competitive against either Moon or Ahn.  So the longer it takes for either Moon or Ahn to back down as the chief alternative to Park means that the race will focus less on Park and more on the Moon vs. Ahn aspect.

Park, as I’ve written before, is virtually defying gravity in that she’s run nearly a flawless campaign — after rebranding her party from the ‘Grand National Party’ to the Saenuri Party late last year, she led her party from a bit behind to win South Korea’s parliamentary elections in April, notwithstanding the incredible unpopularity of the incumbent, Lee Myung-bak (who Park challenged for the then-GNP presidential nomination in 2006).  Since then, she’s essentially been running victory laps through South Korea, and co-opting the message of the opposition by championing issues that are the traditional turf of the left in South Korea — strengthening the social welfare system and reducing income inequality.  Until either Moon or Ahn drops out, the storyline will be Moon and Ahn, while Park glides, rather presidentially, above the din.

Moon is expected to emphasize employment in his campaign and specifically, government support for small and mid-sized businesses (in contrast to the longtime support for South Korea’s large conglomerate ‘chaebol‘ champions, such as Hyundai, Samsung and LG.  Moon, age 59, is a former human rights lawyer and served as chief of staff in the Roh administration from 2003 to 2008.  He faces allegations of influence-peddling during his time as Roh’s chief of staff, and Roh himself committed suicide in 2009 amid a public investigation into his role in bribery and corruption while president.

A Moon-Park race would, in many ways, take the shape of a broader ideological fight that goes back decades in South Korea — Park’s father, Park Chung-hee, took power in a military coup in 1961 and ruled South Korea until his assassination in 1981.  While many Koreans, including Park’s foes, credit Park Chung-hee’s economic program with lifting South Korea from the developing world into the developed, it came at the price of stifling political dissent.  Park’s current candidacy is dogged by doubts that she has not done enough to distance herself from her father’s record on human rights and democracy.  Roh, and his predecessor Kim Dae-jung, were defined early in their political careers by their opposition to the Park Chung-hee administration and their championing of a more democratic South Korea, and Moon, a top member of the Roh administration, would follow in that liberal line.

Ahn, meanwhile, would pose a more generational threat to Park — instead of litigating the old fights of the 1970s and 1980s (and the 1990s and the 2000s…), Ahn represents a break from the current political system altogether.  Park, who in essence served as her father’s first lady from 1974 onwards after her mother’s assassination, is about as clear a symbol of the current system as anyone else in South Korean politics today.

Having rose to fame as the founder of Ahnlab, the country’s largest anti-virus software company, Ahn draws support disproportionately from younger voters.  His support was instrumental in the Seoul mayoral by-election last year that saw independent candidate Park Won-soon win election (after the DUP candidate dropped out of the race).  In announcing his candidacy, he called for a ‘new politics’ that balances both conservative and liberal approaches to South Korea’s economy and for relations with North Korea.

But Ahn, who may have lost some support by waiting so long to enter the presidential race, also has his critics, who say his business experience is overblown.

None of that will matter so long as Ahn and Moon both remain in the race — with the DUP hesitant to drop out of yet another high-profile race in favor of an independent, and the ‘new politics’ crusader Ahn unable to join the DUP, any alliance would be a jangly peace at best, because neither the DUP nor Ahn will want to have their respective brands tarnished (the former by seeming irrelevant to the presidential race, the latter by tarnishing his anti-politics appeal).

If Moon continues to increase his support at Ahn’s expense, however, it seems most likely that Ahn would drop out in favor of Moon, with perhaps a special role for Ahn in any Moon administration.