One further comment on primaries. Up to a certain point - 1968 if I recall correctly - only a few states actually held primaries. In most states, nominees were chosen by conventions, which is to say by professional politicians. In 1968, the Democrats nominated Hubert Humphrey, whom the Left Wing of the party hated (for reasons which in retrospect were not particularly sound). So there was a great deal of pressure for more primaries and more popular control, and today most (but not all) states use the primary system.

This has had unanticipated results. Candidates nominated in primaries tend to be the most handsome and personable, not the most competent. And as someone already pointed out, the system enables the extreme factions in both parties to control. So does the widespread practice of Gerrymandering, which favors incumbents by allowing them to carve out safe, i.e. partisan districts, so they do not have to please middle-of-the-road voters. In other words, the Left Wing crazies in the Democratic Party assured the triumph of the Right Wing crazies in the Republican Party, and brought us to our present sorry condition.

Federalism, the fact that separate states have separate institutions, is what makes the system so difficult to comprehend. I'm a lawyer; I have to deal with the fact that each state has its own legislation, its own court system, and its own common law. For this reason, I think, American law has very little influence in Commonwealth nations. It's just too hard to grasp.