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 to avoid vote splitting.

Prospects for a possible alliance 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.

The latest survey of 1,500 voters by ratings agency Realmeter put Park at 43.1 percent, surpassing Ahn at 26.7 and Moon at 23.8 percent.

In a hypothetical head-to-head matchup, however, showed Moon on 47.6 percent compared to 46 percent for Park, according to polling agency Research View.

The same poll, which surveyed 1,500 voters, also put Ahn at 50 percent and Park at 45.5 percent in a theoretical race between the two candidates.

Park's Saenuri Party, wary of implications of the impending merger, has branded it as collusion between two men seeking self- aggrandizement.

"It's just a political gimmick to defeat Park," Park's spokesperson Ahn Hyung-hwan said following reports of the merger talks.

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