The UK-educated economist with a patrician air took office after the ruling party dumped his predecessor over a 2008 parliamentary election performance that was the government's worst in its now-56 years in power.
He now confronts a multi-ethnic opposition that smells blood and has gained ground with promises to end rampant corruption and reform controversial policies that favour majority ethnic Malays.
The mild-mannered Najib, 59, has the advantages of incumbency, solid personal-approval ratings, control of traditional media, and his own pedigree as he seeks his first mandate from voters.
He is the son of a Malaysian founding father, hails from the Muslim-majority nation's revered ethnic Malay nobility, and has served three decades in the United Malays National Organisation (UMNO), the country's dominant party.
With pressure rising for greater political space, the UMNO lifer has sought to cast himself as an agent of change through limited reforms including replacing some security laws widely criticised as tools to stifle dissent.
But these moves are dismissed by the opposition as electoral window-dressing and viewed with distaste by UMNO conservatives.
Caught in the middle, Najib has avoided deep reform and opinion polls suggest he has failed to alter his regime's image as an arrogant, corrupt, status-quo force.
"On reforms, he is the emperor without any clothes," said Bridget Welsh, a Malaysian politics expert at Singapore Management University.
Sunday's vote pits the Barisan Nasional (National Front) coalition, one of the world's longest-serving governments, against a three-party alliance led by former UMNO star Anwar Ibrahim.
A thin Barisan victory has been widely predicted, but even that could imperil Najib - UMNO is used to thumping majorities and is keen to recover ground lost in 2008.
If that fails, analysts and UMNO insiders say Najib could face a party leadership fight just like that which brought him to office in 2009.
Najib has seemed destined for Malaysia's political summit.
His father was Razak Hussein, Malaysia's second prime minister and a key figure in securing independence from Britain in 1957.
Najib studied economics in England and in 1976 at age 23 won the parliamentary seat made vacant by his father's death.
He later took high positions at Malaysia's central bank, the state oil firm and in the cabinet, including the defence portfolio. He is also currently the finance minister.
Najib has moved to water down policies that give Malays advantages in business and education but which irk minorities, and claims to have shielded the economy from the global woes with huge public spending and cash handouts to citizens.
"While some may have voiced concerns, ultimately the party has delivered a bold and wide-ranging set of reforms, which have expanded civil liberties and made this government the most open and transparent in its history," Najib said in emailed comments to AFP.
But the prime minister's own reputation has been threatened.
He has been linked to allegations of huge kickbacks in a 2002 purchase of French submarines while defence minister, a case later connected to the gruesome 2006 murder of a beautiful Mongolian woman involved in the deal.
Najib denies wrongdoing, but the episode - one of a litany of UMNO graft scandals - has never been fully explained, and an ongoing probe by French justices threatens to revive it.
Najib's wife Rosmah Mansor is also widely seen as a liability, ridiculed for an imperious demeanour, a reputation for meddling in Najib's work, and allegations of high-ticket overseas shopping forays, which she denies.