South Korea's embattled conservative party cancelled and then reinstated the presidential candidacy of Kim Moon Soo within hours as internal turmoil escalated ahead of the June 3 election.

Saturday's chaotic U-turn, after a failed attempt to replace Kim with former prime minister Han Duck-soo, underscored the People Power Party's leadership crisis following the ouster of former President Yoon Suk Yeol over his martial law imposition in December, which possibly doomed the conservatives' chances of winning another term in government.

Kim, a staunch conservative and former labour minister under Yoon, was named the PPP's presidential candidate on May 3 after winning 56.3 per cent of the primary vote, defeating a reformist rival who had criticised Yoon's martial law. But the PPP's leadership, dominated by Yoon loyalists, had spent the past week desperately pressuring Kim to step aside and back Han, whom they believed stood a stronger chance against liberal Democratic Party frontrunner Lee Jae-myung.

After talks between Han and Kim failed to unify their candidacies, the PPP's emergency committee took the unprecedented step early Saturday of nullifying its primary, cancelling Kim's nomination and registering Han as both a party member and its new presidential candidate. However, the replacement required approval through an all-party vote conducted through an automated phone survey, which ultimately rejected the switch on Saturday night.

"While we cannot disclose the figures, the vote on switching the candidate was rejected by a narrow margin," party spokesman and lawmaker Shin Dong-wook said. Kim, who had denounced the party's attempt to replace him as an "overnight political coup", was immediately reinstated as the candidate and plans to officially register with election authorities on Sunday, according to the party.

"Now everything will return to its rightful place," Kim said in a statement. Kim, 73, was a prominent labour activist in the 1970s and '80s, but joined a conservative party in the 1990s.