The race to see who leads the ANC -- and likely South Africa -- until the end of the decade neared an end Tuesday, as party voting closed and the results were being counted.
ANC spokesman Jackson Mthembu said polls closed shortly after 7:00 am local time (0500 GMT) and the results were tentatively expected by midday (1000 GMT).
The results are likely to show a landslide win for President Jacob Zuma, despite his weak poll standings.
Zuma has been challenged by Deputy President Kgalema Motlanthe, who has run a largely silent campaign that has sometimes appeared more like a protest than a real run at the top office in the country.
General elections will be held in 2014 and despite public anger at the state of the country, the ANC is likely to romp home.
It has consistently received around two thirds of the vote in previous elections since the end of apartheid.
A TNS South Africa poll released on Monday showed Motlanthe's approval ratings at 70 percent, while Zuma polled 52 percent -- less than the ANC's total at the last elections.
"Motlanthe's campaign to simply leave things up to the will of the delegates seems to have failed," said Peter Attard Montalto, an analyst with Japanese bank Nomura.
"It is now clear that he did not accept some deal to remain in the top six with Zuma. This is of course a great shame but a consequence of his lack of campaign style and those who have associated themselves with him."
The winner will be chosen by the ANC's roughly 4,000 senior members, who are gathered in the central city of Bloemfontein for a party conference.
As Motlanthe's name was read out to the 4,000-plus ANC delegates gathered for the conference in Bloemfontein, there were cheers, some boos and rival motioning and singing.
Zuma supporters flashed a two-fingered salute to call for a second Zuma term, while Motlanthe supporters rolled one hand over the other in a gesture calling for change.
Despite the long odds, Motlanthe doubled his bet on Monday, refusing to have his name considered for the deputy president post.
So a loss will see him relegated to the political wilderness of the party's backbenches.
He is likely to be replaced by Cyril Ramaphosa, who formally threw his hat into the ring on Monday.
South Africa will now be looking to the size of Motlanthe's vote to gauge the level of discontent with Zuma's rule from within the ANC.
A "Zumanami" could take some of the heat off the president.
Since taking control of the storied party in 2007, Zuma has been embroiled in a series of scandals, prompting some members of the ANC to call for leadership change.
Criticism of his administration reached a crescendo earlier this year when police killed 34 striking miners in one day and it emerged that around $27 million (21 million euros) of taxpayers' money had been used to refurbish his private home.
Thanks to the ANC's insuperable electoral advantage, a Zuma win opens the door to him staying in power until 2019.