South Africa's ruling ANC and its scandal-tainted leader Jacob Zuma are expected to secure a landslide victory when voters cast their ballots on Wednesday, but with the party's trajectory in serious doubt.

Zuma, whose first five-year term in office has been plagued by corruption, mismanagement and often deadly social unrest, made a final nostalgia-tinged pitch to voters on Sunday, promising more economic power for non-whites.

The African National Congress has won every general election since the advent of democracy in 1994 by a landslide and is expected to win by a wide margin this time round too. And the party that controls the legislature picks the president.

According to a recent Ipsos poll the ANC is set to garner 63 percent of the vote, just three percentage points less than in 2009.

During the campaign the ANC has benefited from the outpouring of grief over the death of its former leader and anti-apartheid icon Nelson Mandela as well as celebrations of the 20th anniversary of the first all-race elections.

But the ANC's current leader has proved to be an Achille's heel.

Opposition parties from both the right and left have pummelled Zuma over the $23 million of taxpayer funds used to "upgrade" his private home.

"The ANC has become arrogant because they believe that the voters will carry on voting for them, whatever they do," said Democratic Alliance leader Helen Zille at a rally on Saturday.

"Well, they are in for a big shock on Wednesday."

The alliance is predicted to increase its share of the vote by nearly six percentage points to 22 percent and to do well in major urban centres.

But most voters appear ready to put the party they know before the president they mistrust.

On Sunday 90,000-plus jubilant and defiant ANC cadres packed a cavernous Soweto stadium in a pre-election show of force, during which Zuma at times appeared to be an afterthought.

There were no humiliating boos like Zuma suffered in the same stadium during Mandela's memorial service in December, but the 72-year-old president's lengthy speech got a lukewarm response, with tens of thousands filing out of the stadium as he spoke.

- Bloodied but victorious -

With corruption scandals, poor public services and a cratering economy the ANC will emerge from the election victorious but bruised.

Low turnout is likely to inflate the ANC's share of the vote, which is likely to fall for a second consecutive election.

Former stalwarts like Ronnie Kasrils, a leading ANC veteran, have gone so far as to publicly ask voters not to back the party that delivered them from the white-minority rule of the apartheid era.

Both the DA and Julius Malema's firebrand Economic Freedom Fighters are likely to continue to tap into voters' anger that 20 years of democracy have not improved their lot.

South Africa remains one of the most unequal on earth and sees an estimated 30 demonstrations a day against appalling public services.

An ANC leadership battle may also be in the offing.

Under South Africa's constitution, Zuma's second term would be his last and he risks becoming a lame duck as would-be successors jockey for position ahead of a 2017 elective party conference.

Zuma's promise of more action to redistribute economic power away from the white elite, is unlikely to inspire investors already rattled by social unrest and more populist rhetoric.

Malema's party is expected to get around five percent of the vote at its first attempt after promising to nationalise industry and give poor blacks land currently owned by whites.

"A lower-than-expected majority for the ANC would probably be regarded positively by markets, as it may jolt the party into reforming itself internally over the coming years," said Shilan Shah, an economist with Capital Economics.