The main opposition party on Wednesday was grappling with a mass exodus of non-mainstream faction lawmakers, as many of them questioned the authenticity of its candidate nomination process for the April 10 general election.

Five-term lawmaker Sul Hoon announced he would leave the Democratic Party of Korea after being placed in the bottom 10 percent of the party leadership's performance review for legislative activity. The leadership has claimed they made fair decisions on candidate nomination based on the performance review.

“I have decided to leave the Democratic Party after 40 years,” said Sul, adding, "I have been placed in the lower 10 percent bracket after criticizing the omnipotent Lee Jae-myung.”

Sul has been viewed as an aide to former Prime Minister Lee Nak-yon, who himself left the Democratic Party in December last year to launch his own political party.

He is the latest to hop on the bandwagon of non-Lee faction lawmakers who have been either quitting the party or stepping down from key leadership roles in response to conflict over the party’s candidate nomination process. Observers have been witnessing a widening intra-party rift between the pro-Lee faction and the rest ahead of the legislative election.

Four-term lawmaker and Deputy National Assembly Speaker Rep. Kim Young-joo, Rep. Lee Su-jin and Rep. Park Young-soon also announced their departure from the party the previous day.

Democratic Party Rep. Ko Min-jung on Tuesday announced her resignation as a member of the party’s Supreme Council, claiming that she believes the party should openly discuss its election candidate nomination process.

The non-Lee faction has claimed that the leadership has made biased decisions in the candidate selection process by choosing more Lee faction members to run in the election. Several non-Lee faction members said that they were placed in the bottom 10 or 20 percent on purpose when the leadership assessed them for legislative activity, giving them a disadvantage in the nomination process.

Lee Jae-myung, meanwhile, brushed off the mass exodus during a party policy event held Wednesday. "It is their freedom to join the party as well as to leave the party," he said.

In line with the growing complaints of the leadership's election-related decisions, a former presidential chief of staff under the previous Moon Jae-in administration on Wednesday called on the main opposition party leadership to reconsider its recent rejection of his nomination for the upcoming election, amid escalating tension within the party.

“I request politely and earnestly to the party leadership that they reconsider their nominee choice for the Jung-Seongdong district,” Im Jong-seok said in a press conference held at the National Assembly.

The leadership recently tapped Jeon Hyun-heui, the former chairperson of the Anti-Corruption and Civil Rights Commission, to run in the Jung-Seongdong district, instead of Im who had represented the district for two terms.

“I strongly believed that (the Democratic Party of Korea) Chair Lee Jae-myung would follow through his pledge (with Moon) to make the party a melting pot (of the liberal bloc),” he added.

He was referring to Lee’s visit to Moon's home in the southeastern city of Yangsan early this month, where the two vowed unity within the main opposition party.

Im added that he would “decide his next step” depending on the leadership’s next decision, hinting that he could follow several other members outside of Lee's faction and leave the party.