South Korea's two liberal presidential candidates held their much-awaited talks Tuesday over fielding a single candidate against their conservative archrival, a watershed moment in the run-up to the December presidential election.

The meeting between Moon Jae-in of the center-left main opposition Democratic United Party and independent Ahn Cheol-soo came after much courting by Moon, who has long called for a merger of their respective campaigns.

Prospects for a possible merger between the two men are seen as a game changer that would determine the fate of the presidential election, as they are seen as liberal alternatives to Park Geun- hye of the conservative ruling Saenuri Party.

"We both share the belief that we should join our forces not only to form a union but also to achieve a change of power and political reforms that would follow," Moon, 59, told reporters before the closed-door meeting.

"We will not forget people's aspirations for new politics," Ahn, 50, also told reporters.

Until a day ago, Ahn, a political novice widely popular among the moderates in urban areas, had been deftly deflecting questions about the possible union and instead demanding the Democrats carry out political reforms.

A handful of key figures of the Democratic United Party, including nine Moon loyalists, have stepped down in an apparent attempt to meet Ahn's demands for political reform.

Park, meanwhile, continues to lead opinion polls despite damaging controversies over the legacy of her father, late military strongman Park Chung-hee.

Her Saenuri Party has branded a possible alliance between Moon and Ahn as collusion between two men seeking self-aggrandizement.

The election is slated for Dec. 19, with incumbent President Lee Myung-bak constitutionally barred from seeking re-election.