system
A chat on subreddit /r/NoStupidQuestions
human
We had a really good run between World War 2 and 9/11, where we either believed, or were pressured to believe, that some things are more important than partisanship.

Between 9/11 and 2014, we were sorta-kinda holding it together. But by 2015, that run finally ended for good.
gpt
To add to this, back in the 80s, you had Liberal Republicans and Conservative Democrats so it was more common to see people cooperating across the aisle. Started to fall apart in the 90s.