A new class of voter is waiting to shake up our politics. In 2016, we learned about the crucial importance of “Obama-Trump” voters. Some estimates held that up to 13 percent of Trump’s voters in 2016 had voted for Obama in a previous election. This reflects the moodiness of swing voters, the changing composition of the two parties, and the continuities between the two very different men. Notably, Obama and Trump both ran their campaigns as critics of NAFTA. Hillary Clinton lost about one out of every four Obama voters who were white men with only high-school education. Some analysts...