Demography is destiny, we are often told, and rightly—up to a point. The American electorate is made up of multiple identifiable segments, defined in various ways, by race and ethnicity, by age cohort, by region and religiosity (or lack thereof), by economic status and interest.

Over time, some segments become larger and some smaller. Some prove to be politically crucial, given the political alignments of the time. Others become irrelevant as they lose cohesion and identity.