BANGKOK — Mahathir Mohamad, who long dominated Malaysian politics as prime minister and leader of its governing party, said on Monday that he was quitting the organization because he was embarrassed by its role in protecting Prime Minister Najib Razak from accusations of corruption.

Mr. Mahathir, 90, said that he had no plans to start a new party to rival the one that he was leaving, the United Malays National Organization, and that he was not asking others to quit.

“I want to leave UMNO because it is no longer UMNO,” he said. “It is a party dedicated to protecting Najib. I can’t be a member of such a party.”