Traditionally the ANC president would also become the national president in elections scheduled for 2009 because of the party's grip on South African politics.
But this year's race pits Zuma, the ANC deputy president, against Thabo Mbeki, the country's president, who cannot by law run for re-election as national president.
Ties to the left
Zuma, who has no formal education, rose through the ranks to become head of intelligence in the ANC, a post that gave him leverage over allies and opponents alike.
Zuma's ties to the left have raised fears he could steer South Africa away from policies that have made the country a model of emerging market success.
The race for the party leadership, to be decided between December 16 and 20, puts South Africa in uncharted territory and comes as the rivalry between Mbeki and Zuma has plunged the ANC into one of the worst crises in its history.
Tiyani Rikhotso, ANC spokesman, said the party had not received official nomination results, and Mukoni Ratshitanga, Mbeki's spokesman, declined comment.
But South African media said Zuma was way ahead of Mbeki.
702 Talk Radio said Zuma had won the nomination of ANC branches in five provinces while Mbeki captured four.
Zuma secured 2,270 votes to Mbeki's 1,396.
Although Zuma won easily as expected in his power base KwaZulu-Natal province, he secured an upset in South Africa's financial heartland Gauteng province, receiving 263 branch nominations compared to Mbeki's 94.
The influential ANC Women's League backed Zuma in a tight vote on Monday, in a further blow to Mbeki who has pushed for more women in national leadership positions.
The party's Youth League nominated the ANC deputy president in a separate vote last week.
Branch nominations are an important indicator of who may be ahead, but the race could take an unexpected turn if senior delegates disagree with branches in a secret ballot.
There is also the lingering uncertainty over whether Zuma will be recharged in an arms-corruption case.
Mbeki fired Zuma as deputy president in 2005 over allegations he was involved in the corruption case.
That case was thrown out on a technicality but a court ruling earlier this month opened the way for prosecutors to recharge Zuma.