UNDER PRESIDENT Jacob Zuma, South Africa’s quality of governance has steadily deteriorated, along with its economic prospects and role as a continental leader. A big reason is corruption: Mr. Zuma has allowed graft to flourish and has himself been the target of numerous allegations of financial misconduct, including using public funds for the upgrade of his house. Now, with two years remaining in his second and final term, Mr. Zuma has taken a step that could accelerate the country’s descent — and ensure that it continues even after he leaves office.

In a midnight stroke on March 31, Mr. Zuma removed the biggest remaining check on his government’s excesses by firing Finance Minister Pravin Gordhan. Widely respected in international financial circles, Mr. Gordhan had opposed Mr. Zuma’s unaffordable scheme to buy nuclear power plants from Russia, a nontransparent deal that could open new channels for corruption. He also clashed with a powerful business clan with close ties to the president. Markets reacted quickly: South Africa’s currency plummeted, and the ratings agency Standard & Poor’s downgraded the government’s bonds to junk status.

Mr. Zuma backed down when the firing of a previous finance minister produced such a reaction, but now the political stakes for him are bigger. The ruling African National Congress is due to pick a new leader later this year who most likely will be its candidate for president, and the presumptive favorite, in the 2019 election. The president is hoping to install his former wife Nkosazana Dlamini-Zuma, the recently departed head of the African Union Commission, as his successor; that would have the effect of perpetuating his politics of patronage and, perhaps, shielding him from eventual prosecution.

The ouster of Mr. Gordhan may allow Mr. Zuma to consolidate control of his control over the ANC before its congress. Though several senior leaders protested the cabinet shake-up, including deputy president and potential presidential candidate Cyril Ramaphosa, the party appeared to side with the president last week when its secretary-general called the dissent “a mistake that will not happen again.”

In fact, Mr. Zuma’s power grab is a mistake that could be fateful to South Africa. The economy is already struggling, with growth sinking below 1 percent and unemployment stuck at 27 percent. The removal of Mr. Gordhan will frighten away desperately needed foreign investment and could open the way to bad deals, like the nuclear plants from Russia, that South Africa cannot afford.

It’s still possible that Mr. Zuma could be checked. The president is unpopular, and the influential trade union federation has called for his resignation. The cabinet shake-up prompted protest demonstrations around the country, and there is speculation that Mr. Zuma could be forced out of office before his term ends.

For now, however, it looks as if Mr. Zuma may succeed in entrenching his sleaze-tinged power structure in what was once the party of Nelson Mandela. That would be a tragedy not only for South Africa but also for the cause of democracy across the continent.