MANILA, Philippines — Philippine President Rodrigo Duterte is expected to strengthen his grip on power in midterm polls next week, experts say, clearing a possible path to restoring the death penalty and advancing his pledge to rewrite the constitution.

Duterte has found international infamy for his foul-mouthed tirades, but remains hugely popular among Filipinos fed up with the country's dysfunction and elite politicians.

He has pledged to bring back capital punishment for drug-related crimes as part of a deadly crackdown on narcotics in which thousands of alleged pushers and users have already been killed.

Duterte's tough-on-crime platform—which also includes lowering the age of criminal responsibility from 15 to 12—was key to his landslide election victory in 2016.

Among the 18,000 posts up for grabs on Monday are half of the seats in the upper house Senate, which has stopped cold some of Duterte's most controversial policy initiatives.

Opinion polls suggest administration loyalists are strong favorites to capture the Senate and keep control of the lower House of Representatives.

Duterte has also pledged to rewrite the constitution, which would open avenues to prolonging his power beyond a legally-mandated single term that ends in 2022.

Any change of the nation's constitution, however, would require lawmakers' backing as well as popular approval in a referendum, a high bar that has stymied reform attempts by earlier presidents.

Capital punishment and constitutional reform bills have both sailed through the lower house with little opposition since 2016, but were halted by the Senate's mix of administration opponents and independents.

"The Senate has acted as some sort of institutional check on the worst instincts of the president," political analyst Richard Heydarian told AFP—warning, however, that could change in the midterms.

An opening for Duterte

Historically, the nation's 24 senators—who serve six-year terms—have had a reputation for being more independent-minded than the lower house.

The main opinion poll from Pulse Asia, based on face-to-face interviews with 1,800 likely voters, has key administration supporters taking the majority of the 12 open Senate seats.

Imee Marcos, daughter of deceased dictator Ferdinand Marcos, is expected to win one and thus extend the family's remarkable political return in the decades since it was chased from power by a 1986 popular uprising.

The first enforcer of Duterte's drug war, former national police chief Ronald dela Rosa, is also well positioned despite some of the crackdown's most well-known abuses happening on his watch.

Duterte has thrown some of his strongest support behind former presidential aide Christoper "Bong" Go, who could serve as a direct link between the president and Senate if he wins a seat.

The death penalty has a twisting past in the Philippines, having been outlawed in 1987, reinstated six years later and then abolished again in 2006.

A capital punishment bill that passed the House in 2017 allowed execution in cases where suspects were caught with 500 grams (about 18 ounces) of marijuana, or 10 grams of cocaine, heroin or ecstasy.

Steven Rood, an expert on Philippine elections, said even taking into account the Senate's individualistic reputation, the current poll numbers point to a runway for Duterte.

"I think it makes more likely that some of the president's ideas will get through," Rood told AFP.

Duterte has pitched constitutional reform as a way to create a new decentralised federal republic where regions would be empowered to fix local problems and spur economic growth.

But the various initiatives launched under Duterte have also included proposals dropping term limits, allowing the president to run for another term and weakening checks on presidential power.

Critics have voiced concern the president could be aiming to extend his time in office, although Duterte regularly says he has no desire to stay on past his term.