South Korea's conservatives dropped their presidential candidate on Saturday and reopened the nomination process, as the former prime minister joined the party to challenge the liberal frontrunner in an election just four weeks away.

The People Power Party said its members decided to cancel the nomination of Kim Moon-soo as it struggles to reach a consensus on a single candidate for the snap June 3 presidential vote.

Liberal Democratic Party candidate Lee Jae-myung has been a clear frontrunner to replace conservative former president Yoon Suk Yeol, who was removed from office in April for violating his duties when he declared a short-lived martial law in December.

Kim, who was selected as the conservatives' candidate in a party convention a week ago, has resisted a push by the party to have him step aside in favour of former prime minister Han Duck-soo. Kim said he was the sole legitimate candidate chosen under a democratic process.

His bid to stop the party from reopening the nomination process was denied by a court on Friday, clearing the way for the conservatives to vote on the two choices this weekend.

Kim's camp said the party's decision to cancel his nomination was "against the Constitution, party charter and common sense of any human being".

Kim denounced the party's decision as a "political coup" and accused it of violating internal procedures by replacing him, vowing to take legal and political action.

"Last night, democracy within our party died," Kim said.

"I will immediately take legal and political action against the illegal and unjust replacement of the candidate.

"Those responsible for this situation will be held accountable legally and politically."

Han, the ex-prime minister who served as acting president after Yoon's ouster, joined the race last week, saying his long public service made him the right choice to lead the country to tackle economic, trade and diplomatic challenges.

He joined the People Power Party on Saturday. The party has said it hopes to finalise its new nominee before the national election commission's formal registration for candidacy closes on Sunday.

The feud has overshadowed the conservative party's already difficult battle to retain the presidency while policy debates have taken a back seat. The liberal frontrunner has begun unveiling business policy proposals and national security initiatives on North Korea's military threat.

Both Han and Kim trail the liberal party's Lee by a wide margin in opinion polls.

In two-way race scenarios, Lee has 44 percent support against Han's 34 percent, while he leads Kim 43 percent to 29 percent, according to a National Barometer Survey released on Thursday. (Reuters)