Two front-runners are locked in a tight race to succeed him, embodying starkly different strains within a deeply divided party.
Nkosazana Dlamini-Zuma, a medical doctor and anti-apartheid veteran who served in several roles in previous governments, is also a former wife of Mr. Zuma. She has his support and that of many of his allies, and has adopted his populist rhetoric.
Cyril Ramaphosa, the deputy president, has won the support of some of Mr. Zuma’s fiercest opponents: business groups and middle-class black voters in cities. His own record in business, however — as a former trade union leader whose A.N.C. connections helped him become one of the country’s richest men — has made him a representative of the gulf between South Africa’s tiny new black elite and its poor.
Critics have focused on Mr. Zuma, 75, to explain the A.N.C.’s precipitous decline. And Mr. Zuma, who has six wives and as deputy president was tried and acquitted on a rape accusation, makes an easy target.
But the problems go beyond just one man. Like other liberation parties in southern Africa, including those in Zimbabwe, Angola, Namibia and Mozambique, the A.N.C. has never lost power since ousting white rulers, and has come to focus on retaining that power and the access it provides.
In South Africa — where the economy has stagnated under Mr. Zuma — patronage and corruption have built a system that will be difficult to dismantle. In many of the A.N.C.’s rural strongholds, the party remains the main source of business and jobs.
Nationwide, access to state enterprises has been the reward for Mr. Zuma’s allies, including friends with few professional skills and the Guptas, a wealthy family who have acquired widespread business interests.
State enterprises, through the awarding of contracts, or tenders, have created an entire class of A.N.C. loyalists sometimes derided as “tenderpreneurs.”
“There is nothing exceptional in what has happened to the A.N.C. because it is the path that all African liberation parties have taken,” said Ralph Mathekga, a political analyst. “It has failed to modernize from liberation politics to managing a complex modern society.”
“People personalize it to say it’s all about Zuma, but every post-liberation African society risks having a Mobutu,” he said, referring to Mobutu Sese Seko, the notoriously corrupt former ruler of Zaire, now the Democratic Republic of Congo.
“What makes this different is that people’s expectations of the A.N.C. were higher because it was a latecomer and because of Mandela.”
Apartheid ended in 1994, well after liberation had swept the rest of the continent. Mr. Mandela served as South Africa’s first democratic president from 1994 to 1999. His successor, Thabo Mbeki — whose Pan-African vision was encapsulated in his phrase “African Renaissance” — was forced out of power in 2008 before the end of his second term by Mr. Zuma and his allies.
As the A.N.C. and other liberation parties have remained virtually unchallenged, the nation’s fiercest political fighting has occurred inside the governing party. As A.N.C. delegates from across the country converged in Johannesburg, the closeness of the leadership contest underscored the deep fissures inside the party.
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Should Ms. Dlamini-Zuma win — and she has the support of the party’s youth and women’s leagues — most experts predict a continuation of the Zuma era. Analysts point out that Mr. Zuma, who is still dogged by a multitude of corruption charges, would probably be protected under a government she led.
Ms. Dlamini-Zuma has adopted some of her ex-husband’s populist language, railing against “white monopoly capital,” the term used by critics of the concentration of wealth in the hands of white South Africans.
She has said that, as president, she would focus on “radical economic transformation” by redistributing the country’s wealth from whites to blacks and that she did not care about getting the backing of the country’s business groups.
“I’m not surprised white minority capital is not endorsing me,” she said last month.
Ms. Dlamini-Zuma has defended her public record — “I don’t loot government coffers,” she said — but she has said little about the widespread corruption under her former husband.
A victory for Mr. Ramaphosa, in contrast, would be likely to give the economy a quick boost and prevent a further downgrade of the country’s national sovereign debt, which has fallen to junk level because of Mr. Zuma’s efforts to gain direct control over the country’s treasury and other policies that have driven away investors.
Mr. Ramaphosa could also win back some black middle-class voters who, in recent years, have begun abandoning the A.N.C.
Last year, in the greatest shock to the party since it gained power with the end of apartheid in 1994, the A.N.C. lost control over the nation’s largest cities, including Johannesburg, Pretoria and Nelson Mandela Bay, after black middle-class voters disillusioned with Mr. Zuma’s A.N.C. abstained from voting or jumped to the opposition.
A key negotiator in the talks that led to the end of apartheid, Mr. Ramaphosa was Mr. Mandela’s chosen successor as president. But after losing to Mr. Mbeki, Mr. Ramaphosa entered business, where his career has given him a more troubling legacy.
In 2012, in the worst killing of civilians since the end of apartheid, the police shot dead 34 wildcat strikers at a platinum mine in Marikana belonging to Lonmin, a company where Mr. Ramaphosa sat on the board.
An official inquiry into the massacre found that he had tried to intervene with the authorities on behalf of the company, though it eventually absolved him of guilt. Nevertheless, to many, Mr. Ramaphosa became the symbol of an A.N.C. elite that had betrayed the people it once fought for.
Mr. Ramaphosa, who returned to politics in 2012, has pledged to fight corruption. But during the more than three years he has served as deputy president under Mr. Zuma, he remained largely silent on the issue and stood behind the president, though he has tried to distance himself in recent months.
Last month, in a speech on South Africa’s ailing economy, he said, “We must acknowledge that our ability to overcome these challenges has been undermined over the last decade by a failure of leadership and misguided priorities.”
Neither candidate has inspired people the way past leaders have done over the A.N.C.’s 105-year history. Africa’s oldest liberation party, it once captured minds and hearts across the continent.
“For people in my generation, we grew up in the anti-apartheid struggle,” said Owei Lakemfa, a veteran labor activist in Nigeria, where many A.N.C. leaders sought refuge before the end of apartheid. “The A.N.C. held a lot of promise for us then. Now, it does not.”