Seongnam Mayor Lee Jae-myung, one of potential presidential candidates from the main opposition Democratic Party, said Thursday the resignation of former UN Secretary-General Ban Ki-moon may adversely impact the party's current front-runner.

Ban, who had been a leading conservative candidate, was viewed as a real threat for the presidential ambitions of Moon Jae-in, a contender representing the opposition, until Ban decided to drop out Wednesday.

During an interview with YTN Radio, however, Lee said Moon's solid approval rating was based on the progressive voters' anxiety against a strong conservative candidate like Ban.

"Although (Moon) is the No. 1 candidate in the opposition bloc, there seems to be a ceiling to his (approval ratings)," Lee said. "(Moon) is overwhelmingly strong, but he is also stagnant, and it will not be easy for him to expand further."