Ahn Cheol-soo, the likely candidate of the center-left People's Party, has steadily risen in polls as a powerful challenger to Moon, spawning speculation that the election battle could boil down to a duel between the two who merged their candidacies in the 2012 presidential election.

On Monday, Moon clinched the presidential nomination of the Democratic Party with 57 percent of some 1.4 million votes cast in the in-house poll, beating other big-name contenders, including South Chungcheong Province Gov. An Hee-jung and Seongnam City Mayor Lee Jae-myung.

The People's Party is set to pick its flag-bearer during its last primary round for the Daejeon and Chuncheong region Tuesday.

Ahn is likely to lock in the party's nomination as he has collected 72 percent of the votes cast in the previous six primaries.

The conservative Liberty Korea Party named South Gyeongsang Province Gov. Hong Joon-pyo as its presidential nominee last Friday, while its splinter Bareun Party picked Rep. Yoo Seong-min as its candidate last Tuesday. The progressive Justice Party chose Rep. Sim Sang-jeong as its nominee in February.

Fringe contenders include Lee Jae-oh, a close ally of former President Lee Myung-bak, and former National Intelligence Service chief Nam Jae-joon. Kim Chong-in, a former interim leader of the Democratic Party, is said to declare his presidential bid soon. But these contenders are considered long-shots.

The crowded election field is a rarity for South Korean politics that has usually seen the candidates of two or three major parties vying for the country's top elected office in the past elections.

In various polls, Moon has maintained a comfortable lead over all other contenders with his support ratings hovering in the 30 percent range for more than three months, with his nearest rival Ahn posting ratings of less than 20 percent.

One exceptional survey, conducted this week by local pollster The Opinion, however, showed Ahn with his support at 43.6 percent in a hypothetical two-way race, 7.2 percentage points ahead of Moon. Moon's camp called the results "flawed and biased."

Apparently mindful of Ahn's growing challenge, Moon has been ratcheting up his offensive against Ahn.

Moon has framed Ahn as a politician seeking to align with the conservative force tainted by the influence-peddling scandal involving the now-arrested former president.

Moon's accusations were based on Ahn's recent remarks that if elected president, he could consider extending a special pardon to Park should the public support it. Ahn has dismissed the accusations as part of Moon's negative campaigning.

Despite the accusations, Ahn's forte over Moon is his potential to court centrist and even conservative voters, observers said, while Moon might have limits in appealing to those on the other side of the political aisle due to his campaign agenda that draws a clear ideological line against the right-wing bloc.

The possibility of a candidacy merger between Moon and Ahn remains slim at the moment, as Ahn has vowed to fight until the end of the race and opposed the idea of any "artificial" alignment based on political calculations. 

In the face of formidable liberal rivals, the biggest challenge for the conservative candidates comes from within, not without.

The two right-wing contenders, Hong and Yoo, have recently engaged in an increasingly acrimonious war of nerves over their potential merger with the former calling for the splinter party's "return home" and the latter rejecting the idea of being "absorbed" into the larger party.

Political analysts say that it is inevitable for the fractured conservative bloc to mend its schism and stand united in the election seen unfavorable to it. But the two remain poles apart over the terms of their merger.

The Bareun Party broke away from the former ruling party in December following the parliamentary impeachment of Park over the corruption scandal. As a prerequisite for the merger with the largest conservative party, it demanded the removal of party members with ties to the disgraced former leader. (Yonhap)