President Donald Trumphopes to deliver growth above 3% in the coming decade, which would be hard in the best of times. He and some of his fellow Republicans seem intent on making it even harder by putting the brakes on immigration.

It is a basic rule of economics that a nation’s output depends on the number of people it employs and how productively they work. The Federal Reserve, the Congressional Budget Office and most private economists think output will grow a mere 2% per year in the next decade. To beat the consensus,...