Ahn, who had previously been considered a leading mayoral candidate, decided not to run in September, throwing his support behind Park Won-soon. The software mogul on Sunday announced his plans to campaign on behalf of opposition-backed Park.

On Monday, Ahn visited Park's campaign headquarters and handed him a letter that calls for Seoul citizens to vote for Park, saying he believes the liberal candidate can "bring change" to the capital city.

"This election should be about who can create harmony, not conflict, and who is more truthful, and who is talking about the future, not the past," Ahn said in the letter. "I will exercise my right to vote, and I ask voters to join me."

Ahn's move could deal a blow to Na Kyung-won of the ruling Grand National Party (GNP) in the tight race as Ahn is hugely popular among younger generations due to his clean and upright image exempt from the voter distrust that plagues politics as it stands now.

Ahn has even been mentioned as a possible contender for next year's presidential poll.

Ahn, who currently serves as a professor at Seoul National University, had been reluctant to stand at the forefront of the mayoral race amid fierce competition between the rival camps, despite his endorsement of Park during his own withdrawal from the race.

Political watchers say Ahn's backing has turned the Seoul election into a prelude to the 2012 presidential race as the GNP's leading presidential contender Park Geun-hye has been hitting the campaign trail on behalf of Na in the past weeks.

The pundits are, however, divided over the degree of the "Ahn effect" on Park's approval ratings.

"As the approval ratings between the two candidates have remained close, making Ahn's support for Park public could have a great impact (on voters)," said Hong Hyung-sik, director of the polling agency Hangil Research.

Won Hee-woon, a senior researcher at the Korea Society Opinion Institute, argued that Ahn's decision would have little impact as he had already expressed his support for Park in early September when he gave up his mayoral bid.

"(His support) will not sharply raise Park's approval ratings. It will rather play a role in collecting swing voters who distanced themselves from the voting amid negative campaigning," Won said, referring to the aggressive campaigns between the rival parties over the candidates' ethical qualifications.

Political parties also showed keen interest over whether Ahn could prove to be a game-changer or just a one-time phenomenon.

The GNP played down Ahn's move, criticizing the opposition camp of relying on outside help to run its campaign.

"If (Ahn) wants to enter politics, I hope he throws away his career as a professor and joins politics," GNP leader Hong Joon-pyo said.

Rep. Sohn Hak-kyu, who heads the main opposition Democratic Party (DP), expressed hope that Ahn's backing would unite Park's supporters as well as swing voters disillusioned by the ruling party.

Na stressed that the race should not be a proxy battle but one between the candidates.

"I think the 'Ahn effect' has been already reflected in polls, and there is no special meaning," Na said in a radio interview. "This election should be a race between Park Won-soon and Na Kyung-won. I don't think people will give more votes to a candidate who is backed by somebody else."

The post of Seoul mayor has been vacant since Oh Se-hoon of the GNP stepped down in August after losing a referendum he called to oppose a free school lunch program supported by the opposition-led city council.