When local authorities decline to honor ICE detainers, they can have any number of good reasons for doing so. A likely one is the Fourth Amendment, which forbids imprisoning anyone without justification. If a police department is about to release someone who posts bail, it can’t prolong the detention — in essence, arrest that person again — just because ICE asks it to. Federal courts have repeatedly ruled that the local police cannot be forced to honor a detainer in violation of the Constitution. That is, without an arrest warrant from a judge. Which an ICE detainer is not.

Beyond the constitutional problems lies an argument about public safety, which also finds the Trump administration on the wrong side of the facts, in service of a campaign of fear. Mr. Trump has been trying to make Americans fear unauthorized immigrants. He has succeeded in making these immigrants terrified of him, having declared open season on the undocumented, in effect making every one of 11 million people a priority for deportation. Nobody — not parents of citizen children, not students, not those with clean records and deep American roots — is above suspicion or safe from arrest.

That fear has had palpable effects. Chief Charlie Beck of the Los Angeles Police Department announced on Tuesday that Latino immigrants had suddenly and sharply become less willing to report crimes. He said reports made by immigrant Latinos of sexual assault had dropped 25 percent in 2017 through March 18, compared with the same period last year. Reports of domestic violence fell by 10 percent early this year.

Chief Beck said Mr. Trump’s deportation crackdown had made immigrants afraid of going to the police or cooperating with courts. “Imagine a young woman, imagine your daughter, your sister, your mother,” he said, “not reporting a sexual assault, because they are afraid that their family will be torn apart.”

And now, with his ICE detainer bulletins, Mr. Trump wants local law enforcers to be afraid of him, too. He wants them to fear being publicly blamed for crime by immigrants, to have second thoughts about releasing anyone who might give the administration an excuse to brand them as complicit.