The tough-talking Philippine President is best known for his bloody war on drugs. The Government says more than 5000 drug dealers and users have been killed in police operations. Human rights groups say the toll is likely significantly higher - up to 20,000, including low-level drug users and even ordinary citizens, some in vigilante-style killings related to the drug war.
The Midterms will also determine the independence of the senate, at a time when Duterte is gaining influence over institutions meant to check the presidency.
Although senators have increasingly been allying themselves with Duterte, the body has been able to block key legislation introduced by the President, including measures to reinstate the death penalty, lower the age of criminal liability, and create a federal form of government - proposals that have been condemned by human rights groups and political watchdogs.
The senate has been "the last bastion of resistance," said Aries Arugay, a political scientist at the University of the Philippines.
Administration allies are expected to dominate voting for the House of Representatives.
Though the opposition is politically credible, Arguelles said, they remain "electorally uncompetitive," still widely perceived as elitist, compared to Duterte's image as a brash everyman.
Candidates endorsed by Duterte - who enjoys high approval ratings - have consistently topped polls. These include his former special assistant, Bong Go, former police chief and drug war enforcer Bato dela Rosa, and Imee Marcos, daughter of the late dictator Ferdinand Marcos.
Duterte's campaign in 2016 was widely seen as aided by online disinformation and trolls. Social media analytics show that Filipinos spend the most time online of any population in the world, with a daily average of more than 10 hours on the Internet.
The unprecedented weaponisation of social media for political campaigns caught the Philippine Commission on Elections off guard.
Politicians across party lines have rushed to join the cyber war, hiring online hands with fake accounts to boost their platforms, defend their people and sometimes smear their rivals.
The elections commission has introduced rules in an attempt to regulate campaigning online. Candidates now are required to disclose all their official online accounts. Thirty days after the election, Facebook and other platforms are expected to submit reports of paid advertisements.
"We had an experience that totally caught us flat-footed," said James Jimenez, a spokesman for the elections commission. The new rules, he said, wouldn't solve all problems, - especially since he has a team of only ten monitoring the use of social media advertising - but will "make a dent."
Analysts doubt this will be enough.
"You can come up with rules and regulations," Arugay said. "But if you don't have infrastructure, or political will to implement it, it has no value."
Jonathan Ong, a sociologist at the University of Massachusetts who has studied the troll industry, says plenty of campaign propaganda can still fly under the radar.
"We all know trolling doesn't happen on the official accounts. It happens on the alter[native] account, the anonymous account," he said.
Duterte has also thrown a curveball at the local press ahead of elections, publicising two "matrices" that claim independent fact-checkers are linked to the opposition, the communist party, and people suspected of accusing Duterte of being a drug lord.
The President's office has not provided proof for its claims, but the charts have fuelled online hate at the people named.
Facebook's fact-checking partners, Vera Files and Rappler, are among the accused.
"We continue with the same vigor despite attempted distraction from [Duterte's office]," said Vera Files President Ellen Tordesillas. The nonprofit is part of tsek.ph, a fact-checking initiative among local news organisations and universities.
Arugay believes this new toxic atmosphere online, and the proliferation of false accounts, has become the new normal.
"The mobilisation of trolls we see now - it's a Pandora's box," he said. "We can't bring that back. It will stay."
