He is going up against a record number of candidates and battles in several states, and the results will determine his political future.
Malaysians went to the polls today in the most closely fought election in years, pitting an opposition alliance headed by the country's former strongman ruler against a government led by his scandal-plagued former protégé. The latter politician is leading the charge from prison after being thrown into jail for sodomy in 2014 - an incarceration he says was politically motivated.
Independent pollster the Merdeka Center had predicted that the Barisan Nasional coalition will win 100 of the 222 seats in parliament, while Mahathir's alliance will win 83.
Dr Mahathir said that counting in many constituencies has finished.
In Australia, hundreds of Malaysians gathered on Monday in Melbourne's city center to hand postal ballots to volunteers preparing to fly them back to Kuala Lumpur before the deadline.
"But the most important thing is for the people to decide on the destiny of this nation, and it must be based on facts, it must be based on policy, it must be based on who can execute the best plan for the nation and for the people".
The opposition alliance Pakatan Harapan (PH) had been seeking to motivate people to turn out to vote to "take down this kleptocratic government".
Najib, who is defending the Pekan parliamentary seat, believed that the people will not vote based on slander or mere emotions. A 13th state, Sarawak, has already held its election.
Malay complaints over rising costs and stagnant wages have been growing louder in recent years, especially in the countryside, and after the introduction of an unpopular sales tax in 2015.
Najib has promised to nearly double cash handouts to low-income households.
Despite the 1MDB scandal - the fund is being investigated for money-laundering and graft in at least six countries - corruption issues are not expected to hurt Najib's chances.
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The abuse often happened during what were supposed to be romantic encounters, but the women said the violence was not consensual. After joining the Nassau County district attorney's office, she was appointed chief of the newly created Special Victims Bureau.
He has vehemently denied all allegations and been cleared by Malaysian authorities.
"This is Najib's last stand", said Professor James Chin, a political scientist at the Asia Institute, University of Tasmania.
But this once powerful coalition has seen its popularity decline.
"It's been a calm election in terms of the physical aspect of the election", he told reporters.
But critics have raised concerns the election will not be free and fair.
Peter Mumford, an analyst with Eurasia Group, a political risk consultancy for investors, said the credibility of Malaysia's institutions has been "torn to pieces" by Najib's election tactics.
Mahathir's victory was surreal for his family.
Momentum for the opposition has been gathering, and experts believe that will narrow the margin of victory for Najib. But it could still win a majority in Parliament due to an electoral system that gives more power to rural Malays, the party's traditional supporters. "And that I could exercise my responsibility as a Malaysian citizen".
Najib, the eldest son of the former Premier Abdul Razak, is the candidate of the ruling coalition National Front (Barisan Nasional).
Election officials were yet to disclose the final turnout, although analysts said it could be lower than at the 2013 election, when it was 85 percent.
The opposition is gaining ground in the ruling party's eastern stronghold of Sarawak state, and TV networks report that the heads of a Chinese party and an Indian party within the Malay-dominated ruling coalition lost their seats in regions that are also vote banks for it.