The 65-year-old strongman smiled as he held up an inked finger at a polling booth on Sunday morning in an election devoid of his only serious opposition.
Hun Sen is part of a small coterie of world leaders to hold power for three decades, adapting to the shifting political landscape in the poor Southeast Asian country since the Cold War.
His ruling Cambodian People's Party is set to win big on Sunday after the main opposition Cambodia National Rescue Party (CNRP) was dissolved by the Supreme Court in November 2017.
In his final campaign speech days before the poll, Hun Sen was in typically bombastic form, bragging about the legal action to "eliminate traitors who attempted to topple the government".
Critics say victory will be the culmination of years of state-sponsored violence, intimidation and deft legal footwork by Hun Sen to head off an opposition which emerged as a serious threat at the last election in 2013.
It will also mark a nadir for Cambodian democracy.
"Few of Hun Sen's opponents have had the combination of ruthlessness, guile, and political acuity that have carried him through repeated cycles of Cambodian history," said Sebastian Strangio, author of "Hun Sen's Cambodia".
He was a cadre in the Khmer Rouge, an ultra-Maoist organisation that overthrew the US-backed government of Lon Nol and killed one quarter of the population from 1975 to 1979.
But to escape ever-deeper purges he defected to Vietnam, returning as their army toppled the Khmer Rouge and taking credit as Cambodia's saviour from a group he once belonged to.
The ambitious former fighter was installed as prime minister in 1985, aged just 32.
- 'Uncle' wants your vote -
Hun Sen lost the first election he ever ran for in 1993, the UN-sponsored vote meant to be the dawn of a new democratic era.
But he seized control in a bloody 1997 coup, a year before the death of Khmer Rouge leader Pol Pot.
He's been accused of stacking the military with fiercely loyal allies, while handing key security roles to relatives and his sons in an attempt to build a dynasty.
China has in recent years turned Cambodia's patron ladling out cash and soft loans for infrastructure that left Hun Sen less reliant on a critical West.
A visible presence across the country, he travels frequently to the provinces, giving straight-talking speeches riddled with warnings that without him Cambodia will crumble.
As times change he has remodelled his approach, spreading his message via a Facebook page with 10 million "likes" -- on a platform that could have served the opposition.
He has even co-opted garment workers who once marched with the opposition, raising wages for the near three quarters of a million people employed in the key sector.
"I love him the same as my parents," said garment worker Phoeung La during a recent visit by the premier to nearly 25,000 labourers in Kampong Chhnang province.
Ever-attuned to populist opportunities, Hun Sen handed out $5 in envelopes, while pregnant employees were given $200 by a premier who called himself "uncle" to the crowd.
"You ask for schools from uncle, you ask for roads from uncle, so uncle asks for votes back," he said.
- Genius of survival -
The ruling Cambodian People's Party has won every election from 1998 onwards.
But frustration with corruption among a young population with little memory of the horrors of the Khmer Rouge era gave an opening to new challengers.
Formed in 2012, the opposition Cambodia National Rescue Party (CNRP) won more than 44 percent of the vote in 2013 and almost the same in local 2017 polls.
But their emerging threat was snuffed out.
Sam Rainsy, who co-founded the CNRP but now lives in self-exile in Paris, said Hun Sen's years in the Khmer Rouge were formative in creating a culture of violence and impunity which defines Cambodian power.
A nemesis through endless thwarted years of opposition, Rainsy strikes a reluctant note of admiration for Cambodia's enduring strongman.
"Hun Sen's strength, I would even say his genius, is to survive," he said.//AFP