Tempo,
I'm no historian, but as I understand it, American politics has ALWAYS been dirty and very geographical..... I recall a bit a story about some very early presidential election here, one of the first, and the "mud" was worse than now. And we have had - at times - VERY divisive politics - certainly during the slavery debate and issue. BUT.... talking to my parents and older generations, they CLAIM (at least) that politics in the 50's was much more polite and cooperative, they CLAIMED politics was more civil and that elected folks at least tried to work together. I get the impression the "divide" and extremism we see today has its roots in the 60's and has suddenly taken a turn for the worse. To me, the political atmosphere here is pretty toxic..... in my work and social relations (even at church) everyone has to be very careful what they say. ALSO, "popularism" (a sort of "anti-goverment" approach to government) is DEEP in the DNA of the USA (it's how we began) and popularism rises up periodically in our history - but rarely does this get all the way to the White House as it did 3 years ago (Ronald Reagan had a bit of this, but he very much played within the rules and worked WITH the Republican Party).
Perhaps another thread could address Brexit. In my former job, I got to travel a bit to Europe (I was there several times) and got to talk about European matters..... I also have a very close friend who lives in Spain and some relatives in Europe.... and my wife's family is very Scottish with connections there. I've under the impression that the unification of European countries (begun perhaps 500+ years ago) was motivated by military and economic reasons: bigger meant stronger in political, military, economic terms - and so local identities simply became secondary or even lost (along with their languages). The creation of the EU in some ways is the zenith of this but ALSO the unraveling of this. TODAY the impression I've gotten is that those ancient, regional identities are being reborn and embraced anew (in Spain this is really seen in Catalonia for example). Reality seems to be that military and economic things really don't depend on boarders anymore; a small country (like Denmark) can thrive and be secure in spite of being small. Iceland enjoys a very high standard of living and military security with only about 150K people. I've heard voices that Scotland should be independent from UK BUT be in the EU and Nato, etc. I don't sense this in the USA but then we just don't have those very historic, old, separate nations. When the US joined together, the 13 colonies were all pretty new and all British. Anyway.... maybe this "one world" and this "historic identity" tension ebbs and flows or maybe we're just in a "pull back" period of sorts?