Mr. Najib “wants to leave his own legacy,” he said. “But what he does is verging on criminal.”

Mr. Najib has denied allegations of abuse of power and urged patience while the country’s auditor general completes a report on the transactions of the sovereign wealth fund. “If there is any misuse of power, we will not shield anyone,” he told a Malaysian television channel in April. The report is due at the end of the month.

The political combat has transfixed this nation of 30 million people, an officially Muslim country with one of the most developed economies in the region.

The latest round took place early this month when Mr. Najib was scheduled to address a public forum on the questions swirling around his leadership.

When Mr. Najib failed to show up, Mr. Mahathir took the stage. But he had just begun to speak when the police shut him down, cutting off his microphone and escorting him off the stage.

This is the third time in Mr. Mahathir’s career that he has turned on his former protégés, and he succeeded in sidelining the first two. Another former deputy, Anwar Ibrahim, is in prison on charges of sodomy, which is illegal in Malaysia. Mr. Anwar’s five-year prison sentence, affirmed by the country’s highest court this year, was the culmination of trials that began when Mr. Mahathir fired Mr. Anwar as his deputy prime minister in 1998, declaring “I cannot accept a man who is a sodomist to become the leader of this country.”