“I’m driving through these beautiful fields. I want to grab that corn like you’ve never seen. So rich, so beautiful,” Donald Trump told a standing-room crowd last July, at a Make America Great Again “family picnic” in Oskaloosa, Iowa. An obvious applause line, perhaps, but Trump delivered it with the aplomb of a man who had just taken the lead in every national poll. He was speaking to a crowd of about 700 people inside a high-school auditorium, and another 700 or so were standing outside in the overflow section. The appearance of this crowd was, not surprisingly, homogeneous,…