The party's top post has been vacant since former leader Park Jie-won resigned in May to take responsibility for the humiliating defeat in the presidential election. The new leader's term is to last until early 2019.

Four-term lawmaker Chung Dong-young and former co-chair Chun Jung-bae have already announced their election bids. Two-term lawmaker Lee Un-ju and Moon Byeong-ho, a former member of the party's Supreme Council, are also seen as potential candidates.

The two-day candidate registration for the leadership contest is set to begin next Thursday.

After the allegations erupted in late June that a party member fabricated an election-season tip-off against President Moon Jae-in's son, the party saw its popularity tumbling to the lowest point since it was launched with a rallying cry -- "new, clean politics" -- early last year.

On Monday, the prosecution concluded the party's grandees, including Ahn, were not involved in the case. But it charged two former election campaigners with violating the election law by disclosing false information during the election campaign earlier this year.

The foremost priority for the new leader will be to overcome the scandal and regroup the party ahead of the gubernatorial and mayoral elections next June, which are seen as a public referendum on Moon's first year in office, observers said.

Ahn served as a co-chair of the party for some four months from February last year. He stepped down in June over a corruption scandal involving several party members. (Yonhap)