> Even if one's "guy" is in office, I don't think we feel the same about it the way we did even as recent as Obama, and his administration was when we really began to see the social cracks forming.

I really don't see it that way - this isn't at all a new phenomenon. What feels different to me is the intensity and pervasiveness of division, but then again that could be attributed to being more generally aware of things thanks to the Internet.

I'm 39 and have lived in America all my life. In my childhood, I dimly recall bitter rows over whatever Regan was doing - my parents lost friends over their opposition to him. In the 90s, Clinton scandals were all people could talk about for a good while. George W, of course, took social division to new heights - or, at least, I was finally old enough to appreciate just how far-ranging the dividing effects of ideology can be.

I remember in my first job in the mid-2000s, my team was already socially-clustered based on political/religious identity - "Libs" weren't invited to some parties, and vice-versa.

Of course, Obama's election really triggered a certain section of society, there's no denying that, but all that is to say that none of this is really new. In my view, our "tribes" broadly descend from pro- vs. anti-slavery camps, and the different geographic targets of migration from different parts of England and Europe. (I highly recommend the book Albion's Seed (https://en.wikipedia.org/wiki/Albion%27s_Seed), which explores the influence of regional cultures of 18th-century Britain on American cultures).