Philippine President Rodrigo Duterte during his State of the Nation Address at the House of Representatives in Manila on July 22. (Noel Celis/AFP/Getty Images)

DESPITE MOUNTING international criticism, Philippine President Rodrigo Duterte has accelerated attacks against critics of his murderous “war on drugs,” further accelerating the country’s slide toward authoritarianism.

On July 18, Philippine police filed incitement to sedition, libel, cyber-libel and obstruction-of-justice charges against Vice President Leni Robredo and 35 other critics of Mr. Duterte for allegedly plotting against the president’s leadership. It was the strongest step yet in the regime’s efforts to unlawfully silence its critics. Ms. Robredo is a particularly high-profile target: She has long been outspoken about the president’s bloody war on drugs and is next in line if Mr. Duterte loses his position. If authorities do not drop the police charges, Ms. Robredo and the others could face up to six years in prison.

As Mr. Duterte continues to stack the government with his supporters, crack down on the opposition and silence the media, he is poisoning the already ailing democracy of his nation. The convening of a new and largely pro-government Congress and the president’s call to reinstate the death penalty have only added to the climate of intolerance.

Since his election in 2016, Mr. Duterte’s so-called war on drugs has killed more than 6,600 narcotic users and dealers. Activists estimate that more than 27,000 mainly low-income and marginalized individuals have also been caught in the crossfire. Only one of the murders has resulted in the conviction of the police officers at fault. Now, for criticizing this shocking record, Ms. Robredo and other critics may face criminal prosecution.

They deserve international support. Despite a fierce counter-campaign from the Philippines, the U.N. Human Rights Council voted on July 11 to open an investigation into allegations of extrajudicial killings, forced disappearances and arbitrary arrests. Democratic governments should seize this opportunity to hold Mr. Duterte and his regime accountable. Even if Mr. Duterte chooses to rebuff the investigation, the message to him should be clear: His lawlessness will not be swept under the rug.

Having withdrawn from the Human Rights Council last year, the United States was not involved in the decision to launch an investigation. But that does not excuse the Trump administration from action. Given the long-standing alliance between Manila and Washington, the United States has unique power to place diplomatic and political pressure on Mr. Duterte. President Trump’s support for the Philippine president and tolerance of his abuses have not paid off: Mr. Duterte has merely pocketed the president’s pass and pivoted toward China.

It’s time for the Trump administration to adopt a new approach toward Mr. Duterte. A good first step would be to demand that the charges against Ms. Robredo and other opposition leaders be dropped.