Malaysia's opposition alliance named former premier Mahathir Mohamad on Sunday as its prime ministerial candidate for a general election this year, although the authoritarian who ran the country for two decades and is now 92 years old faces an uphill task.
Mr Najib's coalition lost the popular vote in that election, but won a majority of the seats in parliament.
"In my opinion Tun Mahathir would only use his position as a stepping stone to prepare his son Datuk Seri Mukhriz as PM, as history has shown he fired his deputies for power when he was PM".
The decision to pick Mahathir as candidate for premier was endorsed by senior leaders of four-party coalition Pact of Hope at their convention in Shah Alam, just outside Kuala Lumpur, to rousing cheers by hundreds of supporters.
"Additionally Dr Mahathir's promises of a wholesale reform of the Malaysian political system is somewhat questionable, seeing as it was he who laid its groundwork to begin with", he said, as quoted by the Malay Mail Online.
Merdeka Centre's director Ibrahim Suffian said the Mahathir-led group was at a disadvantage due to a split in the opposition and the redrawing of electoral boundaries that critics say favour the ruling government.
If the opposition wins, the parties said they will ask for a royal pardon for Ibrahim so that he is eligible to become the prime minister.
"I owe and thank Anwar's family for putting behind what had happened to save Malaysia", Mahathir wrote in a post on Facebook.
Wan Azizah Ismail, Anwar's wife, was named the deputy prime minister candidate. He is scheduled to be released in June, according to a government statement.
This is the first time anyone has given an actual date for Anwar's release after serving a five-year sentence for sodomy.
Anwar was once a protege of Dr Mahathir, and the rising star of Malaysian politics, but they had a falling out in the late 1990s.
The displeasure with Mr Najib has brought bitter rivals together - the more liberal Mr Anwar and the authoritarian Dr Mahathir.
Capital Economics, a research company, said Najib's ruling party is "almost certain" to win the election on the tailwind of a strong economy, the weak state of the opposition and widespread gerrymandering.
Anwar used to be heir apparent to Mahathir until he was sacked in 1998 by his boss over political differences, an episode that deeply divided the country.
The corruption scandal Mr Najib is embroiled in involves a state fund, 1MDB.
Anwar, now serving a five-year sentence for sodomy, is due to be released from prison on June 8, the Prisons Department director-general said on Sunday, barring any intervention from higher authorities.
"I urge my friends to focus on the principles of the struggle we started a decade ago", Anwar said, as reported by TMI.