“The biggest mistake that I have made in my life is choosing Najib,” he told voters last week.
Mr. Mahathir, who will turn 93 in July, united a fractured opposition and attracted ethnic Malay voters long loyal to the governing party.
Mr. Najib, however, had many political advantages, including a strong party organization, greater access to campaign funds and gerrymandered districts that favor his National Front coalition.
Malaysia also has weak campaign finance laws that allow for a flood of election spending without identifying the source of donations or disclosing how the money is distributed.
Mr. Najib has been embroiled for years in a scandal over billions of dollars that disappeared from a government investment fund that he once headed, 1Malaysia Development Berhad.
The United States Justice Department concluded that $3.5 billion from the Malaysian fund was laundered through financial institutions in the United States and spent on items like expensive real estate, jewelry, paintings and the production of movies, including “The Wolf of Wall Street.”
The assets include a $27.3 million diamond necklace received by Mr. Najib’s wife, Rosmah Mansor, and a $3.2 million painting by Pablo Picasso that was given to the actor Leonardo DiCaprio.
The Justice Department concluded that $731 million in government funds was deposited into bank accounts belonging to Mr. Najib. United States officials cited the “astonishing greed” of individuals involved in the scam.