Arturo Lascanas, a former police officer, testified before the Philippine Senate this week that over a period of 20 years President Rodrigo Duterte, then the mayor of Davao City, paid him more than $20,000 to murder almost 200 criminal suspects. Mr. Duterte officially denies sanctioning extrajudicial killings. But he does threaten drugs dealers with death and admits that as mayor he gunned down three men on the streets to show police “if I can do it why can’t you.”

After becoming President last June, Mr. Duterte took his campaign against drug dealers and users nationwide. A combination of police and vigilantes have killed more than 7,000 people, some of them innocent bystanders.

Despite all of this, Mr. Duterte seems to be riding high politically. A Pulse Asia poll released in January showed 83% of the public trusts the President, the highest level of any Filipino politician. But Mr. Duterte’s erratic behavior is losing him the support of the political elite. Officially his administration holds a supermajority in the Senate. Yet the President’s allies tried and failed to block Mr. Lascanas from testifying.

That led the government to adopt desperate measures. On Feb. 23, a regional trial court ordered the arrest of Senator Leila De Lima, the President’s most outspoken critic, on drug charges. She is now being held without bail. Solicitor General Jose Calida also threatened to arrest opposition Senator Antonio Trillanes on unspecified charges related to the testimony of Mr. Lascanas.

The government has retaliated against Liberal Party Senators who defected to the opposition on the Lascanas vote, stripping them of their chairmanships. That helps rein in the Senate’s investigative powers but also will make it harder to pass legislation such as tax reform.

These tactics are reminiscent of Ferdinand Marcos’s dictatorship, and that is no coincidence. Mr. Duterte praises Marcos as the country’s greatest President and allowed his burial in the National Heroes’ Cemetery in November. He has forged an alliance with Ferdinand Marcos Jr. a possible successor.

Aside from the drug war, Mr. Duterte’s main asset as a candidate was his promise to “destroy the oligarchs that are embedded in government.” Instead, businessmen who prospered under Marcos have become prominent supporters of the administration. Last month the government helped out the sugar barons by regulating imports of corn syrup.

The prospect of a return to Marcos-era corruption and authoritarianism is likely to turn public opinion against the President. Turnout at a recent pro-government rally to demand “revolutionary powers” for Mr. Duterte fell far short of expectations. Opposition legislators are preparing a push for impeachment. Mr. Duterte portrays himself as a strongman, but his powers may have peaked.