Ruling conservative Saenuri Party candidate Park Geun-hye and Moon Jae-in of the main opposition liberal Democratic United Party are wooing supporters of absent but still influential third candidate Ahn Cheol-soo.
Ahn, a successful software entrepreneur and highly popular liberal independent candidate dropped out of the running last week and gave his support to Moon, a 59-year-old legislator, former human rights lawyer and chief of staff to former President Roh Moo-hyun, who was president from 2003-08.
Ahn, 50, withdrew his candidacy after talks with Moon about merging their campaigns failed.
Yonhap News Agency had reported polls showing that Ahn and Moon would have split the opposition vote, allowing Park to win.
Park, 60, is the daughter of former President Park Chung-hee, a former junta general who seized power in a military coup in 1961. He was elected president in 1963, a post he held until he was assassinated by the chief of his own security services in October 1979.
Her mother, Yook Young-su, was killed in a failed assassination attempt on her father by a pro-North Korean man in 1974.
Park is aiming to become South Korea's first woman president on the back of her widespread popularity among conservatives and supporters of her father.
Roh, who was under investigation in a corruption scandal, didn't contest the December 2007 national election that was won by the conservative Lee Myung-bak of the Grand National Party, until February the former name of the Saenuri Party.
In 2009 Roh jumped off a mountain cliff after leaving a suicide note on his personal computer.
Moon takes on the mantle of liberals' hopes to wrest political power back from the conservatives.
The race could be close according to public opinion polls this week, with Park appearing to be lose ground to Moon, Yonhap reported.
A survey jointly conducted by the Donga Ilbo newspaper and local pollster Research and Research showed the two candidates neck-and-neck at around 45 percent.
Park's Saenuri party is struggling with financial scandals despite retaining -- only just -- its majority in parliamentary elections in April.
The Saenuri Party won 152 seats in the 300-seat National Assembly, 25 more than the Democratic United Party. Saenuri had 165 seats in the outgoing Parliament against 89 for the DUP.
In August the Saenuri Party was rocked by an alleged money-for-nomination scandal surrounding the parliamentary elections.