South Africa’s governing party, the African National Congress, in a remarkable rebuke, suspended its former president, Jacob Zuma, on Monday, for launching “vitriolic attacks” against the organization after throwing his support behind a rival political party.

Calling Mr. Zuma’s behavior “erratic” and “disruptive” ahead of crucial national elections this year, one of the A.N.C.’s top officials essentially assailed the former president as an agent of “right-wing” forces seeking to stifle Black progress.

“Former President Zuma is actively asserting himself as the figurehead of counterrevolution in South Africa,” said Fikile Mbalula, the A.N.C.’s secretary general, reading from a statement by the party’s top decision-making body.

It was a stunning turn of events for a former freedom fighter who was once imprisoned alongside Nelson Mandela on Robben Island. Mr. Zuma later rose to power as the leader of the A.N.C. and the nation, dispensing populist rhetoric that attracted a fervent following.

“It is an extraordinary characterization, there’s no doubt about it,” Bongani Ngqulunga, who teaches politics at the University of Johannesburg and was a spokesman for Mr. Zuma when he was president of the nation, said of the A.N.C. statement. “It just demonstrates the alienation between former President Zuma and the political party that he served and led.”

The suspension signals a break from Mr. Zuma’s corrosive legacy by party leaders who had spent years defending him against accusations of corruption and wrongdoing, even as his actions eroded public support. It is also a show of strength by President Cyril Ramaphosa, a nemesis of Mr. Zuma’s, as he seeks re-election.

“That nine years was largely characterized by negative, which is what we’re getting out of now,” Mr. Mbalula said of Mr. Zuma’s tenure as president of South Africa.

But the suspension — it is the first time the A.N.C. has taken such action against a former president — may also hold dangers for the party. This year’s elections, analysts and even some A.N.C. members say, could sink it below an absolute majority for the first time since the end of apartheid 30 years ago.

In the weeks since he announced that he would not vote for his party this year, Mr. Zuma, 81, has been drawing huge crowds at rallies for a new political organization, Spear of the Nation, which bears the name of the A.N.C.’s apartheid-era armed wing.

Analysts say the impact of Mr. Zuma’s support of the new party, also known as uMkhonto we Sizwe, or MK, was unpredictable. It might draw votes away from the African National Congress, but it might instead rally its supporters. The election is expected to be tight, particularly in Mr. Zuma’s home province, KwaZulu-Natal.

I don’t expect Zuma to have a huge national following,” said Ongama Mtimka, a political analyst and lecturer at Nelson Mandela University. But, he said, “The A.N.C.’s at a position where it cannot lose any support, no matter how small.”

The suspension letter to Mr. Zuma, obtained by The New York Times, said he had violated his oath of membership and the party’s constitution. It said that he had brought the party into disrepute and that he had collaborated with a political organization that opposes its objectives. So serious were his actions, the letter said, that the party was suspending him immediately.

Mr. Mbalula accused Mr. Zuma of suggesting that the A.N.C. could perpetrate electoral fraud, rhetoric that “riles up a political base to foment social unrest,” he said.

It was not immediately clear whether Mr. Zuma would be allowed to challenge his suspension or face an internal disciplinary process.

Lebogang Moepeng, second deputy leader of the MK Party, said it would meet with Mr. Zuma on Monday night to discuss his response to the suspension.

Mr. Zuma serves as the face of an MK party that has tried to position itself as more radical and populist on issues like land redistribution, but analysts say it will be difficult to disentangle him from the A.N.C. in the minds of many voters. In recent weeks, some party leaders tried to persuade Mr. Zuma to return to the fold, an indication of the sway he still holds.

Mr. Zuma has denounced members of Mr. Ramaphosa’s government as “sellouts,” and said he was campaigning for another party to punish them. “My conscience will not allow me to lie to the people of South Africa,” Mr. Zuma said in a statement read by one of his daughters, Duduzile Zuma-Sambudla, last month.

“All this came as a total surprise to all of us,” Mr. Ramaphosa said of his predecessor’s break with the party in an interview with the public broadcaster, S.A.B.C., on Monday night.

Convicted of contempt for defying a court order to testify before a national inquiry on corruption in 2021, Mr. Zuma is not legally eligible to run for president. A wave of protests after his incarceration led to some of the deadliest riots in South Africa since the end of apartheid.

After spending just two months in jail, he was released from prison on medical parole after his doctors said he was terminally ill and could not complete his 15-month prison sentence. A judge overruled the medical parole. But Mr. Zuma returned to prison for less than two hours, and then was set free under an early-release program that critics say the A.N.C. government adopted to shield its former leader from legal consequences.

Earlier this year, Mr. Mbalula admitted that leaders had lied for Mr. Zuma when an independent watchdog found that he had used state funds to upgrade his compound in KwaZulu-Natal. Even in suspending Mr. Zuma, the party was slow to move against him, said Mashupye Maserumule, a professor of public affairs at the Tshwane University of Technology in Pretoria.

“Zuma is a creation of the A.N.C.,” Professor Maserumule said.

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