The head of the main opposition party quit his post on Wednesday and handed over the top job to a key architect behind President Park Geun-hye's campaign pledge of economic democracy.

"I am sorry for causing concerns and disappointment to the people," Moon Jae-in said in a party meeting.

Moon's party has suffered a factional feud for months following the party's crushing defeat in the parliamentary by-elections last April.

Moon had failed to work out differences with Ahn Cheol-soo, a former co-chairman of the Minjoo Party's predecessor, over how to reform their party, resulting in Ahn's departure.

Moon has since changed the name of the party to the Minjoo Party in an apparent move to improve its image before the April 13 poll.

Moon said last week that he will resign from his post as soon as the new party leadership is settled.

"I will do his best for the victory of the parliamentary elections," Moon said, though he stepped down from the party's top job.

The party is being led by Kim Chong-in, who crafted Park's "economic democracy" pledge.

Kim joined the opposition party earlier this month as the party's chief campaign manager for the parliamentary elections. (Yonhap)