"I think if we had the good fortunes of Mandela for two terms, we would have had a better chance because he was a committed democrat," said Mamphela Ramphele, a onetime anti-apartheid activist.
"I'm not so sure that his successors are committed in the same way that he was."
Ramphele is preparing to formally launch her new political party Agang, which will challenge President Jacob Zuma's ruling African National Congress (ANC) in polls next year.
South Africa has had difficulty making the transition from the culture of liberation that fought against the white minority apartheid rule to an effective democracy, she argued.
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"The journey to democratic politics is a journey we didn't as a country understand we needed to undergo," Ramphele said.
The ANC has been the ruling party since the fall of apartheid and the 1994 elections which made Mandela the country's first black president. It has seen overwhelming victories at the ballot box but is now under pressure because of failures to deliver on promises.
After nearly 20 years of democracy, the country was "on a completely wrong track", she said.
"There is no doubt that looking at where South Africa stands, nearly 20 years after that beautiful day in our history, we are a country which has not lived up to the promise of that dream."
On the labour unrest that is gripping the country's vital mining sector, Ramphele said the close alliance between the ANC and unions had worsened the crisis.
"How do you become an honest broker when one of the parties to a conflict is your ally, it's very difficult to be seen to be unbiased," said Ramphele, who is a former chair of gold mining group Goldfields.
She claimed she discovered last week that the offices of National Union of Mineworkers (NUM) double up as offices of the ANC on the mines.