If America’s political institutions are slowly disintegrating, and it is plausible that they are, it is because they are held together less by rules than by norms. Rules are not always clear, but they are defined by some kind of concrete line and attached to some sort of enforcement mechanism. Norms work differently. They are a shared ethic that rule out certain kinds of behavior on ethical grounds. When they are broken, it is not all at once, but step by step, in a series of incremental, leapfrogging violations by the opposing sides.