Trump’s backward…

and even these would no longer include parents. Imposing these restrictions and ending the diversity visa lottery would cut in half the number of legal immigrants.

It is hard to gauge how much of what Mr. Trump says is meant as a scare tactic and how much he really will demand. The one notion that runs through all he says or tweets about immigration is that it is a door for criminals and terrorists to enter the United States. Yet data studied by the Cato Institute indicates that diversity-visa holders and illegal immigrants, the groups most maligned by Mr. Trump, are far less prone to crime than native-born Americans.

Politicians have wrestled for decades with how to deal with immigrants who are in the United States illegally — now around 11 million people. But immigration in itself has been widely regarded as good for America and for the American dream. The preponderance of evidence shows that immigrants help the economy grow. They are more likely to own businesses or to start businesses than the native-born; of the 87 privately held companies currently valued at more than $1 billion, 51 percent had immigrant founders.

There are questions worth examining and debating about whether the United States ought to admit more skilled immigrants and what criteria it uses to screen applicants. But such a debate can’t unfold in the shadow of Mr. Trump’s threat to imminently expel the Dreamers. So what is Mr. Trump really after?

A Gallup poll last June found 62 percent of Americans support maintaining current levels of immigration or even increasing them. And since the country is at nearly full employment, the timing of these anti-immigrant demands might seem odd. Yet it’s no more odd than the president’s tough-on-crime talk at a time when crime is lower than it’s ever been, or his obsession with Islamist terrorists, even though the Government Accountability Office found that right-wing extremists have committed far more domestic attacks against Americans since 2001. Mr. Trump’s approach seems intended less to rationalize the immigration system than to inflame his core supporters by demonizing nonwhite people, as he did when he disparaged immigrants from nations like Haiti and Mexico while praising Norwegians.

Members of Congress know better, and they are aware that there are sensible measures that would clear the immediate hurdle and produce a bipartisan deal. Senators John McCain, the Arizona Republican, and Chris Coons, Democrat of Delaware, have offered a stopgap bill that would end the threat to the Dreamers while strengthening border security. Nothing about diversity visas or family-based migration, nothing for the wasteful wall.

That makes sense. The way we deal with legal immigration should not be changed without a thorough, honest debate.