Lee's supporters said that if elected, Lee, a heavyweight in the ruling party, could play a key role in setting better relations with the presidential office Cheong Wa Dae.

But some critics raised the need for a shift in generations so the party can remake itself afresh.

"Now, with competent, strong party leadership, we need to buttress the Moon Jae-in government. I will do my best in backing up state affairs by beefing up a consultative meeting among the DP, the government and the presidential office," Lee said in a speech. 

Other candidates are: five-term lawmaker Lee Jong-kul; four-term lawmakers Kim Jin-pyo, Song Young-gil and Choi Jae-sung; three-term lawmaker Lee In-young; two-term lawmaker Park Beom-kye and first-term lawmaker Kim Du-kwan.

Five of the eight candidates are aged in their 50s, drawing attention over whether younger lawmakers could be picked to compete against Lee or the 71-year-old Kim Jin-pyo.

The number of so-called pro-Moon or pan-Moon candidates that will fill the top three posts is also a focal point for the upcoming primary.

Those who have close ties to Moon are called "pro-Moon" and those who are broadly in sync with Moon are dubbed "pan-Moon."

Four candidates, including Choi and Park are "pro-Moon" politicians while three -- Song, Lee In-young and Kim Du-kwan -- are "pan-Moon." Lee Jong-kul is included in the "anti-Moon" group, a minority bloc that opposes Moon.

The new DP chief will be tasked with buttressing the government's drive for reforms as President Moon has entered his second year in office.

The ruling party secured a huge victory in the June local elections and parliamentary by-elections on the back of strong public support for Moon and his policy of rapprochement with North Korea. The DP controls 129 seats in the 299-member National Assembly.

But the support rates for Moon and the DP have declined for five straight weeks amid signs of a slowing economy and bleak job market situation, according to a recent poll by Gallup Korea.

The government has been dogged by uproar from small business owners over steep hikes in the minimum wage.   (Yonhap)