Actually, this divisiveness started in the 1780s with the writing of the U.S. Constitution. It was written in such a way to appease the smaller and more heavily slave holding states so that they would join the new union.
That divide has never really been healed. Part of the divide is a mind-set that it is Ok for some people to exploit other people for their own gain — that the economy is a zero sum game with winners and losers. The other side tends (very imperfectly) to believe that if we all do better, than we all do better.
Trump is a crude, corrupt, ignorant, vain and probably mentally unstable personification of an extreme version of the former point of view. Even during the Civil War there was some degree of respect for the other side. Trump is incapable of even understanding that. As a result his actions conintue to dramatically raise the level of anger, fear and distrust in this country, and to isolate us from the rest of the world.
