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
Sorry, the Reagan years were not part of any good run. The 90' were absolutely not part of any good run. The current trend of party above everything started in earnest in the 80's and had taken hold by the Clinton years. 

Newt Gingrich and the Republicans blocked Clinton's healthcare bill because they knew it would be a landmark win for the Democrats. He has said as much.