Moon Jae-in, the presidential nominee for South Korea's main opposition Democratic United Party, officially proposed Tuesday talks with his rival over fielding a single candidate against conservative favorite Park Geun-hye.

The fate of the December presidential election, observers say, hinges in large part on prospects for a possible merger between Moon and independent candidate Ahn Cheol-soo, who are both seen as liberal alternatives to the ruling Saenuri Party's Park.

Moon has consistently voiced hopes for merger talks since his primary sweep in September, as Park continued to lead opinion polls despite damaging controversies over the legacy of her father, late military strongman Park Chung-hee.

Ahn, a political novice whose outsider image appeals to disillusioned voters, has deftly deflected questions about the possible union and instead demanded the Democrats carry out political reforms before launching any talks for an alliance.

"We can no longer delay merger talks," Moon's spokesman Woo Sang-ho said during a press conference in Seoul. "Detailed negotiations should start no later than early November."

The two candidates should form an alliance before they are required to register with the National Election Commission on Nov. 25, the spokesman said, adding the continued delay "goes against the popular aspirations."

Ahn's aides, while not ruling out a possibility for merger talks, did not offer a definite answer.

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