South Korea's right-wing presidential candidate Kim Moon-soo accused his party on Thursday of trying to force him out and threatened to take legal action just weeks ahead of a snap election slated for June 3.

Kim said he believed the party leadership was trying to "bring him down" in favour of another candidate, former prime minister Han Duck-soo, even though he had been chosen as the party's "legitimate" nominee.

The People Power Party (PPP) selected Kim as its candidate on Saturday through primaries but has since demanded Kim and Han negotiate which one of them would represent the party in the election.

Han, who resigned as acting leader last week to run in the polls, is not a member of the PPP but was prime minister under ousted president Yoon Suk Yeol, who was from the party.

Unifying the conservative candidates is seen as one of the few options in order to stand a chance against Democratic Party frontrunner Lee Jae-myung in the election.

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

Han was the preferred candidate among 53 percent of PPP supporters versus 32 percent who backed Kim.

Conservative leader Yoon was removed from office in April over his shock martial law order, prompting the snap election.

But efforts to form a unity ticket among conservatives have proved difficult.

"The forced unification process that's underway now is a forceful candidate replacement and an attempt to bring me down... so it could lead to legal disputes. Stop it immediately," Kim said of the party leadership.

Kim's supporters filed an injunction to stop the party from holding a convention this weekend to officially pick either Kim or Han as a candidate, Yonhap News Agency reported on Wednesday.

The feud has overshadowed the conservative party's already uphill battle to retain the presidency and policy debate has taken a back seat, while Lee met business leaders to discuss job creation and deregulation and urged North Korea to stop military provocation and come to dialogue.

Senior PPP leaders and Han's campaign are pressuring Kim to come to an agreement before May 11 when the registration for presidential candidates closes.

Kim has rejected the calls and proposed a one-week campaign for each candidate and a public survey afterwards to pick a unified candidate. (Reuters)