Rodrigo Duterte has won a sweeping victory in mid-term elections in the Philippines, further consolidating his power and popularity and paving the way for the introduction of controversial reforms.

The president was elected on a populist wave in 2016 and the results demonstrate how little his popularity has dipped over the past three years, despite him achieving international notoriety over a war on drugs that has resulted in thousands of deaths, and his misogynist and anti-religious public remarks.

Up for grabs in the mid-terms were 12 Senate seats, nearly 300 seats in the House of Representatives and thousands of local posts, including mayor and governor positions. Pro-Duterte candidates scored victories in significant numbers.

Most significantly, nine pro-Duterte candidates won Senate seats, meaning the 24-seat upper house is now firmly under his grip. The liberal opposition did not win a single contested Senate seat.

Facebook Twitter Pinterest The 12 newly elected senators. Photograph: Bullit Marquez/AP

Since his election in 2016, the Senate, which traditionally acts more independently and had a tiny majority who were resistant to Duterte, has been a bulwark against some of the president’s most controversial proposals, including the reintroduction of the death penalty.

But with three-quarters of the contested Senate seats now occupied by Duterte allies, his pathway is clear to introduce legislation bringing back capital punishment, as well as other contentious laws such as the lifting of presidential term limits and lowering the age of criminal liability to 12.

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There are now just four anti-Duterte senators left and one, Leila de Lima, cannot vote because she is in jail on drugs charges she alleges were fabricated to silence her.

Among the more controversial new senators is Ronald dela Rosa, a former police chief who oversaw the implementation of Duterte’s war on drugs. The campaign has officially caused 5,000 deaths, though campaigners say the real figure is more than double that. The international criminal court is currently carrying out a preliminary investigation into whether the extrajudicial killings constitute crimes against humanity.

Aligning with Duterte’s position, Dela Rosa said his “No 1 priority” would be to bring back the death penalty. The Philippines outlawed capital punishment in 1987, reinstated it six years later and then abolished it again in 2006. Human Rights campaigners voiced fears that his election would also mean he would have impunity for any crimes committed as part of the war on drugs.

Also elected to the Senate was Imee Marcos, the daughter of the deposed former dictator Ferdinand Marcos.

Facebook Twitter Pinterest Imee Marcos arrives for the proclamation ceremony on Wednesday. Photograph: Bullit Marquez/AP

On the day the polls opened last week, Duterte said he would take the result as a “referendum” on his administration so far. “It could be taken as a referendum, so that if you agree with me, then you can vote for my candidates or the people I am supporting this election,” he said.

Despite the overwhelming victory, the president was noticeable by his absence on Wednesday night, fuelling further speculation that his health may be in deterioration. Duterte, who is known to have health problems, has not been seen in public for more than a week.

In an attempt to ease the rumours, photos of Duterte at home in the presidential palace were released on Sunday. On Monday, the president’s spokesman dismissed reports that he was hiding anything about the state of his health. It is the fifth time Duterte has disappeared from the public eye without explanation since he became president.