Zuma is expected to become president after the ballot, but the African National Congress risks losing the two-thirds majority in parliament it needs to change the constitution.
A big turnout could strengthen Zuma’s mandate in the face of the challenge from a new opposition party formed by ANC dissidents and after corruption charges against him that were dropped on a technicality.
Zuma appealed to South Africans to vote in large numbers and to give the ruling party an overwhelming majority.
"We anticipate a massive turnout and we expect that the people of this country will once again give the ANC a huge and decisive mandate to work together with them to further improve their lives," he told a last news conference before the vote.
Opposition parties hope to capitalise on growing frustration with poverty, crime and AIDS as well as doubts over corruption.
But the ANC has superior organisation and the credentials of being the party that ended white-minority rule in 1994.
A new threat to the ANC comes from the COPE party. It was formed by politicians who broke with the ruling party after it ousted former President Thabo Mbeki last year over accusations he meddled in the corruption case against Zuma, his rival.
Victory for the ANC looks assured on Wednesday, but it would be hard pressed to get the nearly 70 percent of the vote it won in 2004.
Markets could welcome a reduction in the size of the ANC’s parliamentary majority because it would limit its ability to make sweeping changes, although Zuma has said the ANC has no plan to change the constitution at the moment.
Foreign investors and local businesses are wary that Zuma could shift away from policies credited with delivering years of economic growth because he owes his ascendancy to forces on the left of the party.
But Zuma has given assurances that there will be no dramatic change. Room for manoeuvre is also limited by the heavy impact of the global financial crisis on Africa’s biggest economy.
Zuma told the news conference he would not comment on post-election cabinet appointments, which are being closely watched by markets keen to see that Finance Minister Trevor Manuel retains a strong role.