The Republic of Korea's presidential candidates kick off official campaigning for the Dec 19 election on Tuesday, focusing on floating voters that have sharply risen in number following the sudden withdrawal of independent Ahn Cheol-soo on Friday.

Negative campaigning was also seen to escalate as the presidential election became a two-way battle between conservative Park Geun-hye of the Saenuri Party and liberal Moon Jae-in of the Democratic United Party. The two are still neck and neck with less than one month before the poll.

Combination photo taken from file pictures show conservative Saenuri Party's interim leader Park Geun-hye speaking in Seoul April 12, 2012 (L) and Moon Jae-in, lawmaker of the main opposition Democratic United Party and former human rights lawyer, speaking in Goyang Sept 16, 2012. [Photo/Agencies]

The voter turnout will play a pivotal role in the outcome, "as it remains to be seen whether the younger voters in their 20s, who tend to show low voting rates in elections and had been avid supporters of Ahn, decide to turn up at the polls," said Yoon Hee-woong, a senior analyst at Korea Society Opinion Institute.

"Generally speaking, if the voting rate stays around 67 percent or under, it is more likely to be advantageous for Park, who already has solid support, while anything over 70 percent would be in favor of Moon, depending on how well Ahn helps his campaign."

Both camps plan to start their campaigns with a bang.

On Tuesday, Park is scheduled to attend a campaign launching ceremony in Daejeon, which will be a nationwide event held simultaneously in Seoul, Busan and Gwangju, each to be attended by co-chairmen and key figures of Park's campaign machine, including Rep. Chung Mong-joon, CEO Kim Sung-joo and former judge Ahn Dae-hee.

Under the slogan, "To communicate and listen closely," and with the signature red adorning each event, Park's campaign will highlight her policies. They also plan to allow citizens unsupportive of Park to deliver their thoughts on a stage installed on Park's campaign vehicle as a gesture of embracing all voters, and operate a separate rally team comprised of popular former and incumbent lawmakers. Campaign team sources said Park would be visiting more than 10 rally sites a day.

Moon's campaign starts in Busan, his hometown and constituency. He will end the first day of rallying by travelling back to Seoul for an event in Gwanghwamun in the evening.

Avoiding cookie-cutter rallies, the campaign team plans to reduce the number of stump speeches, instead preparing eye-catching programs participated in by well-known party members and supporters from the culture and arts fields.

They will also promote "guerilla-style" rallies by volunteers comprising of students, women and labour group members.

A total of 290 rally vehicles are being sent to cities to help the campaign, party sources said.

With Park and Moon running neck-and-neck in polls, the swing votes of those in their 40s and those living in Busan and South Gyeongsang Province; the economy and tension with the Democratic People's Republic of Korea, among other variables, are considered vital.

According to a survey released Monday by Media Research and Chosun Ilbo, 56.9 percent of Ahn's supporters switched support to Moon upon the former professor's resignation. Another 43 percent said they would support Park or were swing voters. The overall proportion of swing voters surged from 9.3 percent before Ahn's withdrawal to 16.0 percent afterward.

Renewing their offensives against the opposition flag-bearer, the Saenuri Party continued to highlight the fissures between Moon and Ahn, and questioned the policy failures and security stance of the former Roh Moo-hyun administration.

"The force that is dominating the DUP is trying to return to the past, to the so-called 'Roh Moo-hyun era,'" said Ahn Hyung-hwan, co-spokesman for Park's campaign team.

"DUP's Moon, as we could see in the opposition's unified candidacy talks, is a candidate who can change his words anytime as he pleases for his profit…He had said before that 'the most regretful thing was to have served as the chief-of-staff for the Roh administration.' Is this not betrayal, hypocrisy?"

Co-chairman of the election committee Lee In-jae echoed this sentiment in a radio interview, saying, "The DUP had wanted to achieve a more magnificent unified candidacy in order to capture the hearts of the people, but failed. It was as if a plane made a hard landing."

The Saenuri Party also questioned Moon's earlier comments regarding the Northern Limit Line, the de facto demarcation between the ROK in the West Sea, and his position on the 2010 sinking of the Cheonan ship by the North that some on the left demand reinvestigating.

The DUP hit back at the Saenuri Party and Park over their conservative policies, branding them the epitome of outdated politics and tied to the iron-fisted rule of former President Park Chung-hee, Park's late father.

"The Saenuri Party, which had hoped for the race to remain a three-way contest, are spitting out violent words as the candidacies finally unified. It is time that they change their perspective and think hard about what they can do well. The cooperation and consensus between Moon and Ahn remains valid," said Rep. Woo Sang-ho, Moon's chief of communications.

Rep. Jung Sung-ho, co-spokesman for Moon's campaign, also attacked Park.

"Just the fact that during her 16 years and five-terms of parliamentary activities she has only submitted 15 bills makes it difficult to say Park has faithfully carried out her job and deserved the amount of allowances she got."

"The principles and credibility of Park have already collapsed completely…the economic democratisation that she had chanted trumpeted out to be hollow, while the passage of the laws to help small merchants foundered upon the Saenuri Party's opposition," he added.