Malaysian opposition parties have formed a new alliance, called the Alliance of Hope, but opposition leader Anwar Ibrahim is currently in jail, on sodomy charges that his supporters argue are politically motivated.
Mr Najib's greatest threat comes from his own governing party and in particular Mr Mahathir, who is still regarded as the father of modern Malaysia and highly respected.
This is Mr Mahathir's third campaign against the men he groomed to be the country's next leaders. He successfully ousted his deputy, Anwar Ibrahim, in the late 1990s and also forced his successor, Abdullah Badawi, to step down. Now Mr Mahathir has called on Mr Najib to resign.
Pro-democracy group Bersih has held mass rallies demanding Mr Najib's resignation.
However, thousands of ethnic Malays have also rallied in support of the government saying they want to "uphold Malay dignity".