SEOUL - South Korea's conservatives dropped their presidential candidate on May 10 and reopened a process to pick a new nominee as the country's former prime minister joined the party to challenge a liberal frontrunner less than four weeks before the election.

The People Power Party (PPP) said its members decided to cancel the nomination of Mr Kim Moon-soo as it plunged deeper into a bitter strife in trying to agree on a single candidate for a snap June 3 presidential vote.

Liberal Democratic Party candidate Mr 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 by declaring a short-lived martial law in December.

Mr Kim, who was selected as the candidate in a party convention a week ago, has resisted a push by the PPP to merge his campaign with former Prime Minister Han Duck-soo, saying 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 May 9, clearing the way for the conservatives to vote on the two choices this weekend.

Mr Kim’s camp said the PPP’s decision to cancel his nomination was “against the Constitution, party charter and common sense of any human being”.

Mr 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 PPP on May 10. The party hopes to finalise its new nominee before the national election commission's formal registration for candidacy closes on May 11.

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

Both Mr Han and Mr Kim trail DP's Mr Lee Jae-myung by a wide margin in opinion polls.

In two-way race scenarios, Mr Lee has 44 per cent support against Mr Han with 34 per cent, while Mr Lee l...