America’s political parties are the oldest and third-oldest in the world, and they have competed for votes among a population that has been diverse since colonial times. If you have any doubts about that, consult David Hackett Fischer’s 1989 classic “Albion’s Seed” on how settlers from different parts of the British Isles brought distinctive “folkways” to the different seaboard colonies and the Appalachian backwoods. Since none of these groups has ever come close to constituting a majority of Americans, the partisan competition has usually been fierce and (except for the brief and misnamed “era of good feelings”) unending. And fluctuating....