The parties are fairly weak to be honest. Otherwise, Trump wouldn't have been nominated in the first place and Sanders wouldn't have had two strong runs at the

The parties are fairly weak to be honest. Otherwise, Trump wouldn't have been nominated in the first place and Sanders wouldn't have had two strong runs at the presidency. The voters are self-incentivized to break off into factions. I think what has been slowly changing and coming to a head is that the checks and balances in the American system have been eroded and calcified. When everything is all or nothing and mechanisms for compromise stop being used (i.e. legislation, policy diversity across states), then everything is a Big Deal.