JOHANNESBURG — South Africa’s ruling party moved slowly on Sunday to elect a leader to replace the scandal-plagued President Jacob Zuma, as disputes over delegates’ credentials delayed the vote.

Officials with the party, the African National Congress, said a new leader would emerge on Monday morning at the earliest.

On Sunday, officials also said that 400 delegates had been disqualified, leaving nearly 4,800 to decide who would succeed Mr. Zuma.

Delegates from across the country have gathered at a national conference in Johannesburg to elect the next party leader. Given the A.N.C.’s dominance in Parliament, the winner is almost certain to become South Africa’s next president.