You’ll soon be hearing a lot more from President Trump about the magnificent performance of the U.S. economy. That’s because the long expansion that started in June 2009 is approaching, and will likely break, the all-time longevity record. It’s all thanks to him, of course.

Readers of these pages know that I’ve been critical of the president. But it must be said that his economic policies have been the shining stars in a dismal policy firmament. Like most liberal economists, I did not like the way he structured his tax cuts....