Wednesday's elections are set to test whether the ruling ANC party has staunched a decline in popularity, caused by corruption scandals, lacklustre growth and record unemployment.
The party is still expected to win the vote, but the result will reveal whether its new leader, President Cyril Ramaphosa, can reverse growing resentment among voters.
The elections come 25 years after Nelson Mandela led the ANC to power in SA's first multi-racial ballot, which marked the globally celebrated end of apartheid rule.
Support for the ANC has fallen in every election since 2004 with the party winning just 54% in the 2016 local elections, compared to 62% in the last national vote in 2014.
Ramaphosa, 66, took office last year when Jacob Zuma was forced to resign as president after a nine-year reign dominated by corruption allegations and economic woes.
ANALYSIS: The ANC's last easy victory
Most opinion surveys suggest the ANC will secure nearly 60% of the vote on Wednesday, though one poll suggested its share could slide below 50%.
Ramaphosa, seen as a pro-business moderate, is a former anti-apartheid activist and trade union leader who was Mandela's apparent favourite to succeed him as president.
After being outmanoeuvred in that race, Ramaphosa became a wealthy entrepreneur before serving as Zuma's deputy president.
ANC looks to win big
"Ramaphosa gives hope to the electorate that things can go better," said Susan Booysen, a political analyst at the University of the Witwatersrand in Johannesburg.
"If the ANC had gone into an election under Zuma, it would not have won with an outright majority."
The London-based Eurasia risk consultancy said that an ANC victory with 55% or less of the vote would damage Ramaphosa and strengthen aggrieved Zuma supporters within the party.
"This will be the first time that Ramaphosa's popularity will be measured by voters," it said in a briefing paper.
"He is not aligned with Zuma and is driving an anti-corruption campaign and reform agenda aimed at reversing the damage inflicted during Zuma's tenure."
The ANC has been confronted by deepening public anger over its failure to tackle poverty and inequality in post-apartheid South Africa.
The economy grew just 0.8% in 2018 and unemployment hit a record high of nearly 28% in 2017.
State-owned companies were at the centre of corruption scandals under Zuma, with power supplier Eskom now laden with huge debts and forced to ration electricity to many homes, shop and offices.
Wednesday's election will have 48 parties on a ballot paper, though only the main opposition centrist Democratic Alliance (DA) and the radical-left Economic Freedom Fighters (EFF) are also major political players.
The DA hopes to shed its image as a white, middle-class party as its first black leader, Mmusi Maimane, fights in his first general election since he took the helm in 2015.
According to Intellidex Capital research, the party is forecast to make only marginal gains from the 22% it won in 2014 after struggling to make ground against Ramaphosa - a more widely popular figure than Zuma.
Leftist radicals on the up?
But the radical leftist EFF, founded just six years ago by former ANC youth league leader Julius Malema, is predicted to make major inroads, growing from 6.3% to a forecast 11%.
The party, which appeals mainly to young voters and the poor, has centred its election campaign on its flagship policy of seizing land from largely white owners to give to poor blacks.
Enforced land redistribution has also been adopted as a policy by Ramaphosa's government - alarming some investors.
The provincial elections will also measure ANC fortunes, with the party in a close fight with the DA for control of Gauteng, which includes the capital Pretoria and the economic hub, Johannesburg.
Tens of thousands of supporters attended rallies over the weekend, hosted by the major three parties.
"We are humble enough to admit our mistakes," Ramaphosa told the ANC gathering.
"We have taken decisive steps to fight corruption... The era of immunity is over. We are now entering a period of accountability."
Some 26.8 million voters are registered to vote at 22 925 polling stations.
Early results are expected to emerge on Thursday and an official winner is expected to be declared on Saturday.
The party that wins most seats in Parliament selects the president, who will be sworn in on May 25.
Find everything you need to know about the 2019 National and Provincial Government Elections at our News24 Elections site, including the latest news and detailed, interactive maps for how South Africa has voted over the past 3 elections. Make sure your News24 app is updated to access all our elections coverage in one place.
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