Hong Joon-pyo of the conservative Liberty Korea Party remained in a distant third with an approval rating of less than 10 percent.

And with less than three weeks left until the election, the two leading candidates seem only intent on taking a chunk of the other's share of votes.

Unfortunately, however, they are each other's worst enemy as they once shared the same boat.

Ahn, an IT millionaire-turned-politician, even dropped out of a party primary for the 2007 presidential election, giving his blessing to Moon, who went on to face and lose to the then conservative opponent Park Geun-hye.

Ahn, now the presidential candidate of the center-left People's Party, defected from Moon's liberal Democratic Party only last year, meaning he may know everything there is to know about his liberal contender in the upcoming election.

"Not all regime changes are the same. We must aim for a better regime change," Ahn said while electioneering in Seoul on Monday, accusing Moon of long flexing factional power within his own party.

"We must not entrust the country again to a factional power. (Moon) is serving only his own interests by dividing the people," he added.

Moon, on the other hand, has flatly accused his former ally of taking sides with conservative forces responsible for the massive corruption scandal that led to the March 10 ouster of former President Park Geun-hye.

The presidential front-runner has repeatedly insisted Ahn's victory in the presidential election will only mean a revival of the former administration, citing Ahn's recent offer to consider giving a special pardon to the ousted former president, who was formally indicted this week while in jail.

"The upcoming presidential election is really a match between candlelight vigils and the corrupt old power," Moon has said.

The former opposition leader also argues Ahn has no choice but to ally with such corrupt conservative forces to win the election and also to run the government after being elected, noting his opponent's People's Party controls less than 40 seats in the 299-seat National Assembly.

"How could a small, makeshift party with less than 40 lawmakers ever be able to run the government in a crisis such as now," Moon said Monday, the first of a 22-day official electioneering period before the May 9 election.

The Democratic Party currently has 119 seats, the largest number controlled by a single political party.

Still, Ahn has insisted the party's 119 seats will prove useless due to Moon's interest in his own small faction.

The two are also increasingly engaging each other in an apparent character debate.

Ahn's People's Party, along with the former ruling Liberty Korea Party, was the first to open fire, persistently raising allegations over the employment of Moon's son at the state-run Employment Information Service in late 2006, while Moon was serving as a senior presidential secretary.

Moon and his party gave Ahn a taste of the latter's own medicine, raising questions about the 2011 employment of Ahn's wife as a professor at Seoul National University.

So far, the two have met in at least two rounds of TV debates.

No serious duel has taken place so far as the TV debates also involved three other contenders -- Hong of the Liberty Korea Party, Rep. Sim Sang-jeung of the progressive Justice Party and Rep. Yoo Seong-min of the splinter conservative Baren Party.

At least four more TV debates, including three sponsored by the National Election Commission, are scheduled before the election. (Yonhap)