President-elect Donald Trump says he will move immediately to deport or imprison two million, maybe three million, unauthorized-immigrant criminals. “What we are going to do is get the people that are criminal and have criminal records, gang members, drug dealers,” he said on Sunday on “60 Minutes.”

Like many of his proposals, this one sounds tough and straightforward, but makes no sense under scrutiny and is frightening to think about.

Start with the fact that the target number is made up. There simply aren’t as many criminal immigrants as he imagines. According to rough estimates by the Migration Policy Institute, of the country’s 11 million unauthorized immigrants, about 820,000 have criminal records. About 300,000 of those have felony convictions and are presumably the bad people Mr. Trump is talking about. If he deports those and only those, it will be a remarkable display of law-enforcement discretion, since he said that there were lots of “terrific people” among the unauthorized who might be allowed to stay, “after the border is secured and after everything gets normalized.”