Najib is seeking to retain power at the head of a regime that has ruled Malaysia since independence in 1957, but veteran ex-leader Mahathir’s shock comeback has upended the election race.
Angered by a massive financial scandal that has tarnished Malaysia’s international image, Mahathir has teamed up with an alliance of parties that opposed him when he was in power, and which includes jailed opposition icon Anwar Ibrahim – his former nemesis.
Najib’s ruling Barisan Nasional (BN) coalition is seen as likely to win due to an electoral system that critics say is heavily stacked in their favour, but analysts predict the government will lose the popular vote for the second consecutive election.
Under the Malaysian system, a simple majority of MPs in parliament is required for victory, rather than the popular vote.
The race is expected to be tight however. The opposition alliance has gained ground in recent weeks as Mahathir, who ruled with an iron fist for 22 years, has chipped away at the government’s key support base, the Muslim Malay majority.
Long queues were forming at voting centres across the country, which opened at 8:00 am (0000 GMT). They were due to close at 5:00 pm (0900 GMT) with results expected on late Wednesday or in the early hours of Thursday.
Lim Kok Tong, a 43-year-old voter in the capital Kuala Lumpur, said he was "hoping for change".
"When governments realise that they can be replaced, they will be attentive to their citizens," said the voter from the country’s sizeable ethnic Chinese minority, who typically support the opposition.
In the northern city of Alor Setar, former civil servant Jaafar Hassan, 69, said he had voted for the ruling coalition.
"I know Mahathir. He is a good man but he has become arrogant – this is his biggest mistake," he said.
Najib is under pressure to score an emphatic win after the government lost the popular vote for the first time at the last elections in 2013, and observers say his position as prime minister could be under threat if he does not do well.
Voters in the multi-ethnic country have become increasingly disillusioned by divisive racial politics, the rising cost of living and graft scandals.
The controversy surrounding state fund 1MDB has only added to Najib’s woes. Billions of dollars were allegedly stolen from the fund, which was set up and overseen by Najib. The leader and 1MDB deny any wrongdoing.
But in rural areas the rising cost of living is the main concern, and has hit poor Malays hard, particularly after the introduction of an unpopular sales tax in 2015.
Najib, from the country’s revered ethnic Malay nobility, voted in his constituency of Pekan and said the election had been "quite vicious".
"The most important thing is for people to decide on the destiny of this nation, and it must be based on facts," the 64-year-old told reporters.
Critics have raised the alarm about alleged manipulation of the system to tilt the poll in BN’s favour through a redrawing of electoral boundaries which created constituencies packed with Malay voters, who traditionally back the government.
The opposition was increasingly targeted as the campaign heated up, with police launching a probe into Mahathir for allegedly breaking a controversial new law against "fake news" after he claimed a plane he chartered was sabotaged.
In an eve-of-election address, Najib promised tax exemptions for the young and more public holidays if his coalition wins while in a rival speech,
Mahathir urged voters to dump a government that he says has "destroyed" Malaysia.
Najib has countered Mahathir’s attacks by focusing on the ex-leader’s own authoritarian track record while in office.
Some 15 million voters have been registered to vote at the election and 222 parliamentary seats are being contested. —VNS