The nature of the Electoral College by way of the US Senate. This is quite literally as intended. America is a coalition of states that only really started functioning as a cohesive single unit in the wake of the Great Depression.

The fact that a handful of rural fiefdoms still command as much power as they do in the national congress is a large part of what keeps the country gelled together. If Floridians and Idahoians weren't hooked up to the Federal Cash Transfer pipeline, I suspect we'd see a lot more open reactionary insurrections as Californication of the national legislature moved the country's social policy away from its reactionary roots.