If DA supporters turned out in force on election day, the party could deny the ANC a two-thirds majority and the unfettered ability to change the Constitution, party leader Helen Zille claimed on Friday.

"Turnout will be decisive on Wednesday," Zille wrote in her weekly newsletter.

"The mathematics are simple: the more DA voters turn out relative to ANC voters, the better our chances are of stopping Jacob Zuma from getting the two-thirds majority he needs to change the Constitution.

"In previous elections, differential turn-out has benefited the ANC."

Zille said the Democratic Alliance's tracking system has shown that "if every DA supporter goes to vote", the ANC will get less than a two-thirds majority and possibly even under 60 percent.

"In provinces like Gauteng and the Northern Cape, where the ANC's grip on power is weakest, if every DA voter turns out, we could win enough votes to be able to form coalition governments. In the Western Cape, if every DA voter turns out, we could win the province outright."

Zille accused the African National Congress of being power-hungry and presidential front-runner Zuma of failing to understand the Constitution.

"Make no mistake: if the ANC gets a two-thirds majority with Zuma as President, the consequences will be serious.

"Mr Zuma has no understanding of constitutionalism, which is the bedrock of our democracy. He has often said that the ANC is more important than the Constitution, that opposition parties have no automatic right to exist, and that the powers of the Constitutional Court should be reviewed because judges are 'not God'."

The DA, which launched a "Stop Zuma" campaign this week, believes the ANC plans to use a constitutional amendment bill approved by Cabinet on Wednesday to usurp the power of local authorities, in particular those run by the opposition.