President Jacob Zuma of South Africa is facing a leadership challenge from his deputy, Galema Motlanthe.
A vote will take place at the annual conference of the party, which begins on Sunday in Mangaung.
The African National Congress has gained more than 60% of the vote in each of the national elections since the end of white minority rule in 1994. But it is facing increasing accusations that it is not doing enough to boost economic growth, or to combat corruption.
Mr Motlanthe was nominated by branches in three of South Africa's nine provinces.
The BBC reports that although there has been months of speculation that Mr Motlanthe would run against Mr Zuma, this is the first confirmation that he would accept his nomination. He previously said he was "agonizing" over the decision.
Analysts say Mr Zuma is likely to be re-elected, having majority support in the remaining six provinces.
Mr Zuma was elected ANC leader at its last conference in 2007 following a battle with then-President Thabo Mbeki.
Mr Motlanthe backed Mr Zuma and became his deputy in the ANC and the government. He served for six months as caretaker president after Mr Mbeki was forced to resign in 2008.
The ANC Youth League said it welcomed Mr Motlanthe's candidature with great excitement and jubilation.
The organisation has been at the forefront of the campaign to oust Mr Zuma, arguing that he has failed to tackle poverty and unemployment.
Its leader Julius Malema was expelled from the ANC last year for indiscipline after he attacked Mr Zuma's leadership.
