South Africa’s governing party, the African National Congress, affirmed its decision to expel its former leader Jacob Zuma on Friday, rejecting his efforts to remain in the party.

The party, which has ruled South Africa since the end of apartheid 30 years ago, expelled Mr. Zuma in July, casting him as a traitor after he had formed a rival political organization in a bid to challenge the A.N.C.’s dominance.

In the run-up to a crucial election in May, Mr. Zuma led and campaigned for the fledgling party even as he remained a card-carrying member of the A.N.C.

Mr. Zuma refused to give up his A.N.C. membership and appealed his expulsion last month. He demanded to make his case in person, rather than a virtual meeting. He also wanted the party to make public its internal disciplinary processes. In the past, Mr. Zuma has turned his appearances at political and legal battles into impromptu rallies, with hundreds of supporters following him. The party refused his request, and permanently severed ties with Mr. Zuma this week.

“Mr. Zuma’s behavior exemplifies the highest form of ill-discipline,” the A.N.C. said in a statement.

By establishing and leading a rival political party, the former president had not only shown disloyalty, but had also tried to destabilize the party during a time when it was trying to renew itself, the party said.

The A.N.C. said it would not allow any individual to undermine it, “regardless of stature or historical contribution.”

Mr. Zuma, who had been a member of the A.N.C. since he was a teenager, was arrested by the apartheid police at 21 and served time in prison alongside Nelson Mandela and other political stalwarts. He became president of the party in 2007, but his tenure was marred by scandal and corruption. After his second term as the party’s president ended in December 2017, Mr. Zuma was forced two months later to step down as president of South Africa amid political pressure.

His administration later came under scrutiny from a judicial inquiry into corruption, and Mr. Zuma was sentenced to 15 months in prison for refusing to testify before the inquiry. He served about two months in prison before he was released on medical parole.

Rather than disappear into political oblivion, he swore revenge on the party he had been a member of for over six decades. Mr. Zuma became the leader of a new party, uMkhonto weSizwe, known as the M.K. party, which came in third in the parliamentary election, winning 58 out of 400 seats. It was a testament to Mr. Zuma’s enduring popularity and to South Africans’ disenchantment with the ruling party, which saw its support plummet to the lowest result since the end of apartheid.

Even as his face appeared on the ballot alongside a new political movement, Mr. Zuma insisted that his efforts were to take back the A.N.C. from his political rivals, especially its current leader, President Cyril Ramaphosa.