The interim head of a minor opposition party said Saturday that expelling two lawmakers-elect and two other candidates linked to a primary vote rigging scandal is critical for the survival of Korea's progressive movement.

Unified Progressive Party (UPP) chief Kang Ki-kab said in an open letter addressed to party members that failure to take firm action at this juncture will lead to the extinction of the country's left-wing politics.

He said the actions are in line with the decision made by the emergency committee that has initially called on the people mentioned to voluntarily step down.

"The party had already called on the four members to take action but they failed to do so," he said. Kang had personally met the lawmakers-elect and candidates to persuade them to step down on a number of occasions.

The UPP had called on Lee Seok-gi and Kim Jae-yeon, the two lawmakers-elect, who were both convicted of engaging in pro-North Korean activities in the past, to step down by noon on Friday, but the request was ignored. Two other candidates who can inherit proportional seats have also turned down demands that they step aside.

The party had earlier asked for the resignation of all 14 people who participated in the primary to run for the April 11 parliamentary elections as proportional representation candidates, after widespread irregularities were discovered by an in-house panel.

The ruling Saenuri Party won 152 seats in the 300 seat National Assembly, with the main opposition Democratic United Party and UPP winning 127 seats and 13 seats respectively. The remaining seats were taken by a minor conservative party and independents.

State prosecutors have already confiscated evidence that could shed light on the incident after internal party investigations have stalled due to infighting among factions.

Kang, meanwhile, said the UPP must take concrete steps to reform itself and turn itself into a party that represents the interests of the people and the underprivileged.

The party is set to pick new leaders in late June as it prepares for the upcoming presidential race slated for Dec. 19. (Yonhap)