If you blame supply-side for the current condition, then you might be right. As far as the other (politically) viable options were concerned, I would say its a wash. Supply-side is just Keynsianism for corporatists. I think Clintonomics and W play a much bigger role than Reagan though, in Reagan's day when Volcker was leading the Fed there was at least some sanity, which was reflected in 90's relative prosperity.
Plus, I wonder how much economics has to do with it. Surely something, but I think that is not the whole story.
Generally speaking, I think two things are required for the stability of a state.
1) Equitable distribution of taxes
2) That everyone plays by the same set of rules
#2 is something we should pay attention to. The general sense, is that this is no longer the case. Maybe it never was, but the perception now is that it is not. The Wall St bailouts were the epitome of this. But there all sorts of cases that fuel the perception that gov't treats people quite unequally. Hillary is a case in point, everyone knows that if someone who wasn't a Clinton did half the things she did, prosecutors looking for a resume bump would have indicted them several times already just for giving them a good excuse.
If their business has difficulty, they can't even get credit. But if a politically connected business gets in trouble they get cash. Politically connected businesses can even get the rules written in their favor. Compound that with the trade deals intended to help the political class, that negatively impacts the smaller guys. People can tell that the interests of the gov't and their interests are widely different.
Especially when the anti-them rhetoric comes down from the pro-gov't people. When the rhetoric of the left is a constant barrage against them personally, and as a class of people, it should be no surprise that they come to view the left and the gov't as the enemy. When the right makes promises, then doesn't keep them, and also writes the rules against them or buckles in the face of criticism from the left - same deal. Trust goes down.
When trust bottoms out, and even when you "win" you "lose", then people feel helpless. When people feel helpless, it turns to anger.