IMO, the interconnection of the internet is heavily to blame for how divisive politics have gotten, but I offer a different method of action: Prior to the internet, most people only regularly interacted with people in a relatively limited cultural band. When venturing out of that cultural band, you largely came across people interacting on heightened levels of good behavior, because of in-person hospitality norms. We might have had assumptions based on stereotypes of other groups, where a Bay Area city slicker might think of the average Kentuckian as a bit backward perhaps a bit bigoted, and the Kentuckian might have imagined the Bay Area type as a bit pretentious, but any individual of the other group you happened to meet probably seemed fine.

The internet blew that illusion up. Rather than dispelling stereotypes, transparency confirmed them, to an even greater extent than everyone thought. Urban people got to see first hand that rural areas were full of deranged bigots who are perfectly primed to believe wild lies about foreigners eating people's pets, and the rural people saw that the urbanites really dismissed them out of hand as speed bumps on the path of progress. Paradoxically, both sides had been imagining that the political divide was smaller than it really was.