The maverick politician had repeatedly said he would resign to show he is a "responsible" presidential candidate.

"Ahn has put his all into creating a South Korea where the people win," Son Kum-ju, senior spokesman of the party's election campaign, said in a press briefing. "(We) will build the future with the people and enable the Republic of Korea to dream again."

The move is widely seen as an attempt to undermine his biggest rival Moon Jae-in of the liberal Democratic Party. The two candidates are currently neck and neck in opinion surveys with their combined approval rating at some 80 percent.

Both candidates ran for the 2012 presidential election before Ahn dropped out to back Moon, who then lost to Park Geun-hye of the then ruling party. Moon held on to his parliamentary seat then.

Ahn's resignation reduces the party's legislative seats to 39 in the 300-member National Assembly. The former software mogul had been serving his second consecutive term since winning a by-election in a district in Seoul in 2013.

Ahn and his team marked the start of the 22-day official campaign period by descending on the liberal stronghold of Jeolla where the party holds 23 of 28 Assembly seats.

"Our plan is to turn the tide in Honam (Jeolla) and then seize the chance to victory in the capital area and other regions," said a key campaign official. (Yonhap)