The election competition is increasingly seen as a two-way race between front-runner Moon Jae-in of the liberal Democratic Party and runner-up Ahn Cheol-soo of the center-left People's Party, with their combined support ratings reaching nearly 80 percent.

Other contenders include Hong Joon-pyo of the conservative Liberty Korea Party, Yoo Seong-min of the splinter right-wing Bareun Party and Sim Sang-jeung of the far-left Justice Party. The election is seen as more favorable to the liberal bloc in the wake of a massive corruption scandal involving Park and her longtime confidante.

Moon has said that to address the decades-long nuclear standoff with Pyongyang, he would strengthen the alliance with Washington and deepen cooperation with neighboring countries such as China, Japan and Russia. 

However, the front-runner said in his book, published earlier this year, that though he is supportive of the US, Seoul should learn to say "no" to American demands, an apparent criticism of Seoul's diplomacy that is highly dependent on Washington.

Moon served as a chief of staff to former President Roh Moo-hyun, who was elected in 2003 after riding anti-American sentiment triggered by the deaths of two girls run over by a US military vehicle during an exercise here in 2002.

Roh sought a "fairer, balanced" relationship with the US based on his controversial vision of South Korea becoming a "balancer" in Northeast Asia. The ill-fated vision triggered speculation in Washington that Seoul would remain "neutral" between Washington and Beijing.

Moon's stance on the ongoing installation of a Terminal High-Altitude Area Defense system, a core US anti-missile asset, on the peninsula, had been seen as a source of contention with Washington, until recently when he apparently readjusted his stance on the deployment.

Moon initially said that the deployment had to be delayed for the next government to decide on it. But after Pyongyang's recent missile launches, Moon warned that THAAD would be inevitable should Pyongyang stage another nuclear test or major provocations.

Ahn, the centrist presidential aspirant, has appeared stronger in his support for the alliance, as he pledged to beef up the country's independent military power with "the alliance forming its foundation."

On the THAAD issue, Ahn has remained positive, saying Seoul cannot unilaterally alter the bilateral agreement on the deployment.

But the stance of his party, led by Park Jie-won, a liberal political bigwig, has been seen as a potential hurdle to the bilateral alliance. The party as a whole has opposed the THAAD deployment.

Ahn's party chief Park served as a chief of staff to former liberal President Kim Dae-jung whose soft-line stance on the communist regime and unconditional largess to it once triggered a conflict with the US government. Kim served as president from 1998-2003.

Conservative candidates Hong and Yoo have similar positions on the alliance.

Flaunting security skills as their forte, both have characterized the alliance as a life-or-death issue, vowing to further enhance security cooperation with Washington.

But Hong's aggressive security stance could cause friction in the alliance, analysts said.

Hong has called for the redeployment of American tactical nuclear weapons to the South, which were withdrawn from the peninsula after the two Koreas declared a joint statement on denuclearization in 1991.

Observers said the redeployment could undermine Washington's longstanding global non-proliferation drive, while others raise the possibility that Hong's stance appears intended to secure more security commitments from the ally.

Sim of the Justice Party has consistently opposed the THAAD deployment, a stance likely to cause tensions with Washington.

In a recent TV debate, Sim has also shown his desire for the resumption of dialogue with Pyongyang through a "comprehensive deal" based on a freeze on the North's nuclear program and the suspension of South Korea-US military drills. (Yonhap)