South Korea's former independent presidential candidate Ahn Cheol-soo issued a full-throated endorsement of his onetime rival Thursday, a move that has widely been expected to improve the chances of the main opposition candidate.

Ahn, who recently dropped out of the presidential race after negotiations with liberal opposition candidate Moon Jae-in over fielding a unified candidate to compete with conservative favorite Park Geun-hye fell through, offered his wholehearted support to Moon.

"Today will mark a watershed for the presidential election. I' ll do my best to fulfill people's aspirations," Ahn told reporters after meeting with Moon in central Seoul.

Earlier in the day, the former candidate, through his spokesman, urged his supporters to rally behind the presidential nominee of the center-left Democratic United Party. "Ahn said that he will unconditionally offer assistance and believed that such action would complete the single candidacy process and lead to victory in the polls," Yoo Min-young, Ahn's spokesman, said.

Ahn's endorsement has been expected to raise Moon's chances of winning the Dec. 19 election, which has turned into a close two- way race between Moon and the ruling Saenuri Party's nominee Park after Ahn's withdrawal.

Park has held a small but steady lead over Moon for most of the campaign, but her advantage has been within the margin of error for most polls.

The two contenders have been vying to win over swing voters and Ahn's supporters who analysts say hold a key to the upcoming presidential election.