The retired statesman, who is one of the prime movers in attempts to unseat the prime minister, said Najib ran the risk of being investigated and even imprisoned if he were to step down.
“It is not that he does not want to step down, he is afraid because if he does, no power can stop the investigations and he will be jailed.
“That’s why he does not want to step down,” Dr Mahathir told a crowd of Umno members today at a high tea event organised by the Pusat Bandar Taman Cempaka Umno branch.
Najib is under fire over a RM2.6 billion “donation” from an unknown Middle Eastern country. The money was deposited into his personal accounts, a large portion of which was given ahead of the 2013 general election.
He is also under pressure over his brainchild, state investment firm 1Malaysia Development Berhad (1MDB), which is facing multiple probes, including by foreign authorities, over alleged financial irregularities.
However, Najib had repeatedly stated that he would not step down, and told American fund managers and investors to ignore the “1MDB noise”.
Dr Mahathir today described Najib’s administration as “cruel” for acting against his critics.
He cited the example of sacked Umno member Datuk Seri Khairuddin Abu Hassan who is currently detained under an anti-terrorism law, the Security Offences (Special Measures) Act (Sosma), for lodging reports against 1MDB in other countries.
The police have said that Khairuddin’s actions amounted to using foreign enforcement agencies to put pressure on Malaysia over the 1MDB issue, and could be deemed an act of sabotage against the country.
Dr Mahathir, who led Malaysia for 22 years, also said investigators probing into 1MDB were either sacked or transferred.
He went on to question the identity person who donated the RM2.6 billion to Najib, who has said that it was normal for him as Umno president to hold donations in trust on behalf of the party.
“No person in the world will give you RM2.6 billion because they love you so much. Who is this donor? I, too, want to meet him.
“Not only we do not know who the donor was, but we don’t even know where the money went,” Dr Mahathir said, adding that Malaysians had the right to ask these questions.