http://www.people-press.org/interactives/political-polarization-1994-2017/
Notice that under Clinton it wasn't so bad. When Bush was inaugurated it still wasn't horrible. Perhaps 9/11 had something to do with it, but their graph is from 2004, meaning the Iraq War had started. So who knows?
Other graphs (not this one) show that the split really picked up in 2007. It continued through Obama's election (notice, it didn't start with Obama's win, although that certainly helped push people right and left) and it picked up even more after 2011 (meaning that it wasn't Obama the person). In 2016 the 'lumps' on the Pew charts flattened--the medians stayed about right, but people moved more right or more left, balanced by some flow to the center.
Trump's win also helped split. But much of the division happened not because of Trump, but under Obama.
Much of the polarization is a sorting of views on discrete topics.
http://www.pewresearch.org/fact-tank/2017/10/23/in-polarized-era-fewer-americans-hold-a-mix-of-conservative-and-liberal-views/ (There are other sources, but why be diverse when one source can provide all my information?) In other words, those holding one position are falling into line with all the others holding that position. It's like Snow's two cultures: It was true once that if you did technical, scientific stuff you also did at least some humanistic, artistic stuff. By his day, he could lament that if you did technical and scientific work you were walled off from the arts and humanities; and if you did H&A, you didn't do STEM. The traits and attributes were sorted into two discrete stacks, and each set up their own value system--and, worse, each thought they had the one true way.
My observation: Whenever you force people to do what's "correct" when they haven't bought into it just pushes them away. That can be Christian missionaries trying to convert those with some sort of pre-existing beliefs. It can be political missionaries trying to convert those with some sort of pre-existing beliefs. The missionaries are sure they have the unbridled, ultimate truth, and their views will usher in the millennium. This produces teams, where the goal of winning for the team and the goal of beating the other side are inseparable from making society pure and destroying the infidels. True believers and those who see victory and a benefit buy into it; those who don't, need to be killed, or at least marginalized and made politically inoffensive until they can be assimilated, quarantined, or die off. The goal of the behavior is the same, but we humans only have so many behavioral schemata that we fall back on.
= new reply since forum marked as read
