“The D.A. has to grapple with the perception that it is a white party and it is interested in preserving white privilege,” said Sejamothopo Motau, a member of Parliament for the alliance. “Its leadership has to reflect that the party has gone beyond that — that the party is a party of all South Africans. So having Lindiwe as one of the two leaders of the party at the national level is quite significant.”

Yet one of Ms. Mazibuko’s loudest critics was a member of her own party, Masizole Mnqasela, a member of Parliament, who wrote a letter months ago to the party’s leader, Helen Zille, saying that Ms. Mazibuko “does not have a strong resonance with the black constituencies, and it will defeat our objective of trying to attract black votes.”

Mr. Mnqasela, facing disciplinary charges from his party because of those and other comments, was more supportive in an interview, saying, “It is an opportunity to have a black leader, and especially a black woman — it is an opportunity in itself for the organization.”

Ms. Zille, one of Ms. Mazibuko’s biggest supporters, said it was ridiculous to suggest that Ms. Mazibuko was elected only because she was black.

“Can’t people be judged for who they are?” Ms. Zille asked. “She’s highly intelligent, she’s very capable, she’s extremely articulate, she’s a very good politician and she’s black.”

Ms. Mazibuko has maintained a tough exterior amid the questions about her blackness, saying she has heard them since her childhood. Suggesting that all black people must be a certain way was an apartheid way of thinking, she said. During some of her radio appearances, she said, people called in demanding to hear her speak isiZulu.

“I refuse to do that,” she said. “I don’t believe being black is a prize people can award to you.”

Ms. Mazibuko was born in the tiny kingdom of Swaziland, tucked between South Africa and Mozambique. Her father was a banker and small-business owner, and her mother was a nurse.