Julian Sanchez outlines a cognitive bias:
Nietzsche wrote at length about the tendency to define the good as whatever is opposed by some hated other. Unfortunately, he called it “slave morality”a term I’m not going to touch with a ten-foot pole in the current climateand only some of what he says about it is really applicable in our context, so I’ll use “oppositional morality” instead. Whatever natural instincts we have toward this kind of binary in-group/out-group thinking are probably exacerbated by a political system that ultimately pushes people to pick one of two viable teams, even though this is a poor fit for the variety of worldviews and interests in a large and diverse population. Otherwise incoherent coalitions are bound together by each defining themselves, somewhat circularly, as the negative of the other.