Senior ANC members and KwaZulu-Natal voters should not support any leader facing criminal charges, Cope spokesman Siyanda Mhlongo said on Thursday.

"There are many, many questions that Zuma must answer even before he becomes the face of the ANC in elections," Mhlongo wrote in an open letter.

He was referring to African National Congress president Jacob Zuma, whose criminal case on Wednesday was remanded to August 25, when he was likely to be the president of South Africa.

"Cope... is also worried that if South Africans make a mistake by voting Zuma into office, the Zuma camp in the ANC will do the following -- push for a law to indemnify a sitting president from prosecution, and push out certain officials in the National Prosecuting Authority, to replace them with their cronies who will argue that there is no case for Zuma."

Mhlongo urged ANC leaders "who hate corruption" not to campaign for Zuma.

The party said it was also "disturbed" by the ANC's reaction to the Zuma case, as the party had revealed it intended applying for a stay of prosecution.

"We... are concerned that instead of fighting corruption, the ANC wants to lodge an urgent application to have Zuma's case quashed."

Mhlongo described the ruling party as "immoral" and "not sober".

He said the country's two former presidents, Nelson Mandela and Thabo Mbeki, had not campaigned to become president "with such serious charges hanging over their heads".

Many ANC members, including MECs, were presently facing criminals charges. Those convicted and sentenced, like former ANC chief whip Tony Yengeni and Zuma's former financial advisor Schabir Shaik, were "treated as cheerleaders and celebrities", he said.

"When we were in the ANC, Zuma and his cronies refused [to let] us discuss a conference paper on 'Revolutionary Morality' at Polokwane. Why?" he questioned.

After Zuma was charged, "a sober ANC could have given or forced Jacob Zuma.... to take leave... It wants a person charged with such serious offences to be its face".