Preliminary results suggest candidates endorsed by Mr Duterte have taken nine of the 12 crucial Senate seats up for grabs in Monday’s vote.
The 24-member Senate has been the most important check on Mr Duterte’s power during his three years in office so far. If the unofficial results are confirmed, it would give the president an effective majority in the upper house and allow him to pursue unencumbered an agenda that includes massive infrastructure spending, the return of the death penalty and even re-drafting the constitution.
With his sexist jokes, foul-mouthed public statements and allegations against him of massive human rights violations, Mr Duterte has been seen as a divisive figure.
But analysts said it was difficult to argue with his personal appeal to voters in the Philippines based on these election results. “This president's popularity and transferability of his popularity [to other candidates] is unprecedented to say the least, despite all the controversies,” said Manila political scientist Edmund Tayao.
The opposition had suffered a “wipe-out” in the election, Mr Tayao said, which also included votes for hundreds of mayors, governors and congressmen across the country. It failed to take any of the 12 up-for-grabs Senate seats, the other three appearing to go to independent candidates.
One of the four remaining incumbent opposition senators, Leila de Lima, is also the biggest public critic of Mr Duterte's anti-drugs crackdown that has seen thousands of suspects killed by police. She has been detained since 2017 on narcotics charges.
Presidential spokesman Salvador Panelo said the Senate's independence would not be in question, but the vote showed the public overwhelmingly backed Mr Duterte and his vision.
“Undoubtedly, the Duterte magic spelled the difference,” he said. “People yearn for stability and continuity of the genuine reforms that this administration started. They yearn for a constructive and not obstructionist Senate.”
The profiles of the successful candidates backed by Mr Duterte are a clear indicator of just how fed up the Philippine public is with previous ineffective progressive leaders.
They include Imee Marcos, the daughter of the late dictator Ferdinand Marcos, as well as former national police chief Ronald dela Rosa, who enforced the president's crackdown on illegal drugs.
With the Senate on his side, Mr Duterte could yet enact his most controversial policy yet. He has spoken previously of rewriting the country's 1987 constitution to allow a shift to a federal form of government.
It is a proposal that some critics fear may be a cover to remove term limits. As things stand, Mr Duterte must step aside in 2022 after serving a single, six-year term.
Strict protections against a return to dictatorial rule were written into the constitution after Marcos was thrown out in a popular revolution. Nonetheless, given his already firm grip on the lower house, Mr Duterte could pass a redrafting of the constitution if he can secure the backing of 18 of 24 senators.
The opposition, which has at times successfully fought Mr Duterte’s attempts to roll back civil liberties and accuses him of risking the Philippines’ claims in the South China Sea by pandering to China, said on Tuesday it will not give up. Official results from the Commission on Election will not be released for around another week.
“Our fight for justice, for sovereignty and a more progressive future for our people continues,” said incumbent opposition senator Francis Pangilinan.
Additional reporting by agencies
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