In a radio speech, interim Saenuri leader Park Geun-hye said she feels "deeply sorry" for failing to examine the two candidates before nominating them.

"(The party) should have put all its efforts into improving people's livelihoods, but past wrongdoings by some lawmakers-elect have caused concern," she said.

The apology came after Kim Hyung-tae, a former journalist who won a parliamentary seat in a district in the southern city of Pohang in April general elections, quit the party over allegations he attempted to rape his sister-in-law 10 years ago.

Moon Dae-sung, a gold medalist in Taekwondo who won a seat in a district in Busan, also left the party over allegations he plagiarized a large chunk of his doctoral thesis.

The scandals tarnish the ruling party's upset victory in the tightly contested parliamentary elections held just eight months before the presidential poll.

The surprise win defied previous expectations -- a series of scandals that had hit hard the ruling party will bolster the liberal opposition represented by the main opposition Democratic United Party and the left-leaning Unified Progressive Party.

The election victory also boosted Park's presidential prospects, as the daughter of former military strongman Park Chung-hee remains a top presidential contender on the conservative ticket.

She is credited with leading reform efforts focused on shedding its image as a party for the rich and the powerful, helping the beleaguered party retain its parliamentary majority.

The departure of the two would-be lawmakers, however, leaves the Saenuri Party with 150 seats in the 300-member parliament.

Observers say Park now needs to seek a broad alliance with moderate and conservative legislators to maintain the ruling party 's sway over the two center-left opposition parties, which garnered 140 seats in total.

Park also needs to overcome infighting between her coterie and those loyal to President Lee Myung-bak, her political nemesis.

Some of pro-Lee loyalists, including Gyeonggi Governor Kim Moon- soo and former special affairs minister Lee Jae-oh, are looking to compete with Park to win the party's nomination for the presidential race.

Park is expected to resign from the chairwomanship at a party convention in May.