Rodrigo Duterte answers questions from journalists during a May 10 news conference in Davao City, Philippines. (Jes Aznar/Getty Images)

RODRIGO DUTERTE, the winner of the Philippines’ presidential election on Monday, says he is no Donald Trump. “Trump is a bigot. I am not,” he has said. Mr. Duterte is nevertheless about to offer an example of what happens when a populist demagogue triumphs over a discredited political establishment. The results are unlikely to be good for either the Philippines or its close alliance with the United States.

Mr. Duterte, the 71-year-old mayor of Davao City in the southern island of Mindanao, will succeed a pillar of the old elite, Benigno Aquino III, who has presided over strong economic growth and the upgrading of the U.S.-Philippine military alliance in the face of China’s expansionism in the South China Sea. Despite Mr. Aquino’s success, poverty and corruption remain deeply rooted, and a 39 percent plurality of voters — all that is needed in the Philippine electoral system — bought Mr. Duterte’s big promises of radical changes.

Some are clearly undeliverable: The president-elect said he would eliminate violent crime in six months. Some could be dismissed as hyperbole, such as the pledges to litter Manila Bay with the bodies of criminals and to ride a Jet Ski to plant a flag on Scarborough Shoal, the disputed territory 150 miles offshore seized by China in 2012.

However, there is more than rhetoric to Mr. Duterte’s menace. Human Rights Watch dubbed him the “death squad mayor” for more than 1,000 extrajudicial executions of alleged criminals, including some street children, during his 20 years as mayor in Davao. Far from offering excuses, Mr. Duterte has embraced that record and promised to repeat it on a national scale; he says he will murder suspects himself and then grant himself a presidential pardon.

Notwithstanding his bluster about flag-planting, Mr. Duterte could also undermine U.S. efforts to deter China’s attempts to enforce its far-reaching territorial claims. Mr. Aquino rightly refused to negotiate bilaterally with Beijing over the Scarborough Shoal, instead bringing a case before a U.N. tribunal while concluding a pact that will allow the United States to use five Philippine bases. The two countries recently began conducting joint sea and air patrols.

Mr. Duterte, however, has expressed doubts about the alliance with Washington and hinted that he is ready to strike a deal with China. He even said he would set aside the maritime dispute in exchange for Chinese construction of a rail line in Mindanao. No doubt the regime of Xi Jinping would happily agree.

Philippine optimists say Mr. Duterte’s wild rhetoric was meant to establish himself as a political outsider and offers little indication of what he will do in office. In the past, his acts have contradicted his words; he has spoken crudely about women but promoted gender equality as mayor. It nonetheless appears that the already complicated U.S. mission of mustering counterweights in East Asia sufficient to deter China’s overreaching is about to get still more difficult. Responsible people in both countries can only hope that Mr. Duterte is not soon matched with an American counterpart.