(And yes, conservatives who believe Obama was the sole source of division are also out of touch. People really need to try to understand the other side).
It was like a mirror image of Trump's election in 2016.
While personally I preferred Obama (as a non-US citizen) it's really sad how much anger and rage is expressed over these political differences.
Personally I think gerrymandering is the proximate cause, as it creates more safe districts for party members, which allows them to be more extreme than would be acceptable in a more competitive district.