MARIKANA, South Africa — When Alton Dalasile got his first job as a miner in the late 1980s, he immediately joined the National Union of Mineworkers, a powerful organization that not only fought for workers’ rights but also battled the brutal system of racial segregation known as apartheid. When his union’s political ally, the African National Congress, was on the ballot in 1994 in South Africa’s first fully democratic election, Mr. Dalasile enthusiastically cast his ballot for Nelson Mandela, the country’s first black president.

Last year, Mr. Dalasile gave up on his union amid a violent season of wildcat strikes that ended with 34 miners being gunned down by the police here in August and joined a radical upstart union that accused the old guard of selling out to mine bosses. Now Mr. Dalasile is contemplating what was once unthinkable: voting against the African National Congress in elections next year.

“They have abandoned and betrayed us,” Mr. Dalasile said. “The A.N.C. is no longer the party of the poor man, the working man. They care only about enriching themselves.”

Next year South Africa will hold elections for its National Assembly, which elects the country’s president. Twenty years after the end of apartheid, when the African National Congress swept to power on a wave of international good will, Mr. Mandela’s party is facing perhaps its fiercest electoral challenge yet.