The presidential candidate from South Korea's far-left party quit on Sunday in an attempt to unify left-leaning voters against the conservative front-runner in an increasingly tight race.

Lee Jung-Hee from the United Progressive Party bowed out three days before the December 19 poll in what she called a move to prevent the ruling conservative party candidate from winning.

"I quit today to help liberal, progressive political parties join forces together and fulfil the goal of administrative change," Lee told reporters.

Lee -- supported by only about one percent of voters according to opinion polls -- was a distant third in the race behind Park Geun-Hye from the New Frontier Party (NFP) and Moon Jae-In from the left-leaning main opposition Democratic United Party.

But her departure could offer a slight boost to Moon, who has been neck-and-neck with Park or slightly behind her in recent polls.

Lee used her campaign largely to heap attacks on Park, daughter of the late military-backed strongman Park Chung-Hee who ruled the country from 1961-79.

Widely respected for transforming the impoverished, war-ravaged nation into an economic juggernaut, he is equally reviled in some quarters for human rights abuses during his iron-fisted rule.

"If Park Geun-Hye's party gets re-elected in the presidential poll... it will be an irreversible disaster to all people," Lee said.

The current President Lee Myung-Bak from the NFP is constitutionally barred from serving another five-year term.

The four surveys published last Thursday put Park's lead over Moon as narrow as 0.5 points or as wide as 3.5 points.