* Early tally suggests Duterte to boost mandate
* Opposition expected to struggle
* Senate traditionally vital check on state power
* Official results expected in 4-6 days

Philippine President Rodrigo Duterte's allies held the top spots as unofficial results of a mid-term Senate election trickled in on Monday, signalling broad public backing for the maverick leader.

Early results suggested Duterte-backed candidates would take most of the 12 Senate seats available, with just one from the opposition slate on the fringes.

A Senate majority would lessen the chance of censure moves and lower house probes against Duterte's government and make it easier to pass controversial legislation, such as restoring capital punishment and changing the constitution to introduce federalism, and possibly extend term limits.

The ballot was billed as a referendum on the mercurial president, with the focus on his bid to boost his influence over an upper house vital to him delivering on his reform agenda.

The Senate is just as crucial to his opponents, traditionally a check on state power and a bulwark against the kind of political dominance that Duterte is demonstrating.

Overall, the massive nationwide ballot will decide 18,000 posts, among them more than 200 mayors and governors, half of the 24-seat Senate, and 245 spots in a lower house expected to again be stacked with Duterte loyalists.

However, late into the night, hours after voting ended, only a small fraction of the tally was available due to problems transmitting results. Official results are expected in four to six days.

The midterms come at a time when the 74-year-old former mayor is seemingly untouchable, with last year's spiralling inflation now under control and recent polls showing his Senate candidates scoring highly, and his own public approval rating at a staggering 81 percent.

‘It's not the right environment for the opposition. The opposition could have targeted other candidates instead of making the campaign against President Duterte,’ said political analyst Dindo Manhit.

‘That's a hard battle. That would've been challenging.’

Police reported several minor shootings and bombings in notoriously hostile areas on poll day, and said vote-buying was massive, with more than 200 arrests.

Technology was a problem, with hundreds of instances of electronic voting machines malfunctioning and trouble with the election commission's internet servers.

His popularity has helped to insulate him from criticism over his deadly war on drugs, his misogynistic jokes and insults at the church, and his indifference towards China's rapid militarisation in the South China Sea.

Duterte looks set to consolidate power further in the latter half of his presidency having recruited for his candidates a potent surrogate in his daughter and Davao City mayor, Sara Duterte, in what is being seen as a succession play for the 2022 presidential election.

‘The president is not on the ballot, neither is mayor Sara. That was a wise move on the part of father and daughter, they were willing to use their brand,’ political strategist Malou Tiquia told CNN Philippines.

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