Experts say the dominance of social media space by his administration and his supporters has made Duterte almost untouchable.
Voting was underway in the Philippines on Monday in mid-term elections seen as a referendum on the rule of maverick President Rodrigo Duterte, who looks set to strengthen his grip on power and further marginalize a dwindling opposition.
Nearly 62 million of the country's 107 million population are registered to vote in a massive nationwide ballot for 18,000 posts, among them more than 200 mayors and governors and 245 seats in a lower house in which Duterte is all but certain to retain his huge majority.
The most important race is for the 12 available slots in the 24-seat Senate, a less-partisan chamber until now seen as a crucial check on power and a bulwark against the Duterte administration's political dominance.
Opinion polls indicate that candidates backed by Duterte will prevail, the latest putting 11 or 12 of his allies among the 15 seen as having a chance. Only one opposition candidate was among the 15, demonstrating what analysts say is Duterte's growing clout and an opposition struggling with relevance, cohesion and leadership.
"Duterte's number one objective is to consolidate power... He'll get a big mandate, maybe even possible three-fourths of the Senate," said political analyst Ramon Casiple.
"The opposition may not even get one seat and that would be a record. A complete shutout."
There were no indications of significant violence on poll day, but there were widespread reports of vote-buying and breakdowns in electronic voting machines.
Duterte won the 2016 presidential election by a big margin as an alternative candidate and a no-nonsense newcomer on a national stage traditionally held by Manila elites, powerful dynasties or famous politicians tainted by scandals.
He has built on that to boost the latter half of his presidency, bringing on board household names and recruiting a powerful surrogate in his daughter Sara Duterte, in what is being seen as an early succession move.
A Senate majority and opposition absence would allow his allies to horse-trade with independents, lessening the chance of censure and Senate probes, and making it easier to pass controversial legislation like restoring capital punishment, and changing the constitution to introduce federalism, and possibly extend term limits.
Experts say the dominance of social media space by his administration and his supporters has made Duterte almost untouchable, despite a war on drugs that has killed thousands of people, accusations of misogyny and insulting the church, last year's spirally inflation, and indifference towards rival China's militarization in the South China Sea.
"Crucially, today's vote also depends on where one stands with regard to President Duterte," the Philippines Daily Inquirer's editorial said.
"The outcome of today's exercise depends on our own estimation of how well, or how badly, Duterte rule has been for this country."