Seoul, South Korea (CNN) -- Two South Korean presidential candidates have decided to join forces in an effort to challenge the conservative front-runner in a move that could be a game-changer for the election next month.
The election will determine who will lead South Korea, a key U.S. ally in Asia, for the next five years amid long-running tensions with the unpredictable, nuclear-armed North.
In a closed-door meeting Tuesday, the two left-leaning presidential hopefuls, Moon Jae-in of the Democratic United Party (DUP) and Ahn Cheol-soo, a professor and former software mogul, agreed to field one candidate between them, according to a statement from the DUP.
But a decision on which of the two will run against Park Geun-hye, the candidate for the governing Saenuri Party, hasn't been decided yet, the DUP said. Candidates must register before the end of November.
Park's party called the move by her opponents a "show meant to confuse voters."
The current South Korean president, Lee Myung-bak of the Saenuri Party, is prevented by the constitution from running for another term.
All three candidates are expected to pursue a more conciliatory line toward North Korea than Lee, who took an uncompromising approach to dealings with Pyongyang.
Popular among conservative voters, Park has said she aims to focus on welfare policies. If elected, she would become the country's first-ever female president.
She is a daughter of former president Park Chung-hee, who stirs mixed feelings among South Koreans. Some claim he was a dictator who stifled opposition while others credit him with overseeing a key phase of South Korea's economic development. He was shot and killed by his intelligence chief in 1979.
Ahn, who founded an anti-virus software company, has gained popularity for his criticism of the large South Korean conglomerates, known as "chaebols." He has argued that the nation's economy is dominated by a few rich individuals.
He is a professor at South Korea's prestigious Seoul National University and a former medical doctor.
Moon, a close ally of late president Roh Moo-hyun, is a former human-rights lawyer who was jailed in the 1970s by Park's father.
With 43 days to go until the election, Park is ahead in polls. She had an approval rating of 42.1%, followed by Ahn and Moon who rated 27.4% and 24.1% respectively, according to a poll conducted Tuesday by the research firm Realmeter.
About 1,500 people participated in the phone survey, according to the company.