The architect of Rodrigo Duterte’s brutal campaign against illegal drugs in the Philippines has almost certainly won a senate seat in the country’s midterm elections, prompting concerns among victims’ groups.

Former police chief Ronald “Bato” Dela Rosa is among Duterte’s allies who were on track to take nine of 12 open seats in the upper house, with 95% of ballots counted. The senate has previously been a bulwark against some of the president’s most controversial proposals.

“[Dela Rosa] now has his own influence and clout, independent of the president,” said Kristina Conti, a lawyer for the victims’ group Rise Up for Life and for Rights, which filed criminal and civil cases against police in relation to Duterte’s drug war. “He might use his political clout to whitewash investigations into the human rights violations of the police.”

Before the elections, families who lost loved ones in the drug war agreed not to vote for Dela Rosa. “Now that he’s going to be a senator, two mothers who filed cases against police told me they’re afraid for their security,” said Rubilyn Litao, a church worker helping those affected pursue cases against the police.

More than 18,000 positions were at stake in the vote, primarily local posts, but also half the senate and nearly 300 seats in the lower house of representatives.

Duterte’s deadly crackdown has drawn international censure, but is central to the populist appeal that has buoyed his remarkable standing among Filipinos since taking the presidency in 2016. Administration loyalist candidates for the senate were heading for a resounding win after Monday’s vote.

Another of Duterte’s hand-picked candidates on course for a senate seat is Imee Marcos, daughter of the late dictator Ferdinand Marcos.

Imee Marcos (centre in checked shirt) in Manila last year with her mother, the former first lady Imelda Marcos. Photograph: Francis R Malasig/EPA

She is poised to beat scions of political families who fought her father’s dictatorship, including Senator Paolo “Bam” Benigno Aquino IV. He is the nephew and namesake of Benigno Aquino Jr, whose assassination in 1983 invigorated opposition to the Marcos regime.

No candidate from the opposition senatorial ticket made it into the top 12 in the current tally. A number of the candidates have conceded defeat. Historically, the nation’s 24 senators – who serve six-year terms – have had a reputation for being more independently minded than the lower house.

“My fear is there will be no genuine opposition,” said Samira Gutoc, from Marawi City, an opposition candidate who has criticised the imposition of martial law in Mindanao region.

Gutoc has conceded defeat but vowed to continue her opposition to Duterte and push for the rehabilitation of Marawi, which was devastated by a five-month battle between government forces and Islamic State-affiliated militants in 2017.

“This election just gave Duterte carte blanche to push his brand of governance to its logical conclusion: complete transformation of the nation’s political system,” the analyst Richard Heydarian told AFP.

Senator Francis Pangilinan, the president of the opposition Liberal party, said: “We need to strengthen our ranks as well as our resolve to face the coming challenges.”

As part of his drug crackdown that has killed more than 5,000 people, Duterte has pledged to bring back the death penalty and lower the age of criminal responsibility from 15 to 12.

The Philippines outlawed capital punishment in 1987, reinstated it six years later and then abolished it again in 2006.

He also promised to rewrite the nation’s constitution to create a federal republic in which regions would be given more power to tackle deep-rooted poverty.

However, opponents see those plans as an effort to extend his hold on power or weaken the nation’s democratic institutions.

Agence France-Presse contributed to this report