Something like three-quarters of Americans, surveys over the past year report, think democracy in America is threatened. To go by exit-poll data, those voters supported Trump in about the same proportions as those who thought democracy was secure. In a study published last year, researchers at U.C. Berkeley and M.I.T. provided evidence that democratic back-sliding around the world — with citizens voting for authoritarian leaders — is driven in part by voters who believe in democracy but doubt that the other side does. The researchers found that such voters, once shown the actual levels of support for democracy among their opponents, became less likely to vote for candidates who violated democratic norms. The general point is that not understanding the actual views of people of other parties — and assuming the worst of them — can be dangerous for democracy.