Rep. Kim Tae-ho of the ruling Saenuri Party said Wednesday he will join the party's primary to compete with the leading candidate Park Geun-hye for the presidential nomination.

The former South Gyeongsang Province governor is the fourth contender in the primary scheduled for late August, though former party chief Park is widely expected to clinch the nomination for December's election.

Earlier this week, two other candidates pulled out of the race after the party rejected their demands for revising primary rules that they claim are designed to comfortably select the longtime favorite Park.

"We have to fight against the old leadership that hinders us from pursuing our dreams and new challenges," Kim said in a statement declaring his presidential bid. "We have to fight against the old thinking and old system."

Kim was nominated as prime minister in 2010, but quit amid alleged ethical lapses revealed during confirmation hearings. The 50-year-old re-emerged after he won last year's by-election in late President Roh Moo-hyun's hometown, considered as a liberal stronghold, and was re-elected for the region in the April general elections.

In the ruling camp, Park, the daughter of late President Park Chung-hee, has been holding double-digit leads over her rivals in opinion polls. She officially announced her bid on Wednesday.

Meanwhile, Gyeonggi Province Governor Kim Moon-soo also plans to declare his bid on Thursday, his office said.

The governor has also demanded the party fully open its primary voting to non-party members, and there has been speculation that he may decide against a run in protest of the party's decision. (Yonhap News)