The difference: Sanders supporters are (often admittedly) idealistic, Clinton supporters have fantasies of who their candidate is.
Invoking mental illness has been a controversial thing in this election cycle but I’ll do it anyway: it reminds me of how people with absent or unreliable parents comfort themselves with fantasies of who their parents actually are. As you suggest in your title, they rationalize everything their parents do.
It’s a thing that we all do to an extent, as most of us either have an absent parent, a parent who’s at work for much of our childhoods, or at the very least a distracted parent who was there but gave us minimal attention.
But like all things in childhood, it’s very easy to take it too far, and/or never grow out of it.
I recently read a book that, while not the greatest book and somewhat outdated, does lend insight into this. It’s called The Fantasy Bond. It is at the very least worth reading a synopsis of it. It describes a problematic way of thinking that many of us practice to an extent, and definitely lends insight into how many political movements work.