PHILIPPINE PRESIDENT Rodrigo Duterte poses a difficult challenge for the United States. He is the democratically elected leader of a long-standing American ally whose strategic cooperation is important in checking China’s aggressive expansionism in the South China Sea. But he is also the author of a heinous campaign of extrajudicial killings of suspected drug traffickers and users that has led to more than 7,000 deaths since he took office last June. While a rupture with his government is not in the U.S. interest, tolerating his abuses threatens grievous damage to America’s moral standing in Asia and beyond.

A subtle U.S. policy would recognize the need for U.S.-Philippine cooperation without endorsing the contemptible offenses of the current president. Instead, President Trump has offered Mr. Duterte an unqualified embrace that effectively blesses his murderous campaign. In so doing, Mr. Trump sends Asians the message that there is no difference between China’s amoral foreign policy and that of this U.S. administration.

Mr. Trump’s extraordinary endorsement of Mr. Duterte came in a late-night White House statement issued after a phone call between the two presidents Saturday. The release described the conversation as “very friendly,” adding that “the Philippine government is fighting very hard to rid its country of drugs.” “Fighting hard” is one way — the wrong way — to describe the wanton killing by police and vigilantes of accused dealers and users. It implies that Mr. Duterte’s tactics are appropriate or necessary, which they are not. Mr. Trump ought to have shunned the Filipino leader until he reined in those practices. Instead, he invited him to the White House.

White House invitations have often been withheld by presidents as a way of distancing themselves from unsavory leaders, but Mr. Trump dispenses them indiscriminately. He has already hosted Egyptian strongman Abdel Fatah al-Sissi, and not long after speaking to Mr. Duterte he issued another red-carpet invitation to Thailand’s Prayuth Chan-ocha, who led a military coup against an elected government in 2014 and has since overseen a sweeping crackdown on dissent.

White House officials contended that Mr. Trump’s outreach was needed as part of his mobilization of pressure against North Korea. But the Philippines and Thailand have never played a significant role in the politics of the Korean Peninsula and are unlikely to do so now. The administration might more honestly argue that the Southeast Asian states are needed to help counter the Chinese regime of Xi Jinping — only Mr. Trump has lately been touting his good relations with Mr. Xi and appears reluctant to say anything that might offend the Chinese Communist leader.

What must be particularly disturbing to U.S. democratic allies is that, even as Mr. Trump was heaping goodwill on the likes of Mr. Duterte and Mr. Prayuth, he was trashing a South Korean government that has been a critical U.S. partner. That drive-by may very well wreck the administration’s effort to bring more pressure to bear on North Korea, regardless of what Southeast Asian nations do. Meanwhile, Republicans who faulted President Barack Obama for disrespecting U.S. allies while courting rogue regimes ought to ask themselves if Mr. Trump is not outdoing his predecessor.