I’ve always liked lines, demarcations, borders, no trespassing zones, so now I want to go back and find the line crossed where things became untenable politically, when the polarization permanent-iscized. Certainly the election of Trump was the signal event at which there was no going back. November of 2016 might’ve marked the rubicon crossing.
But ultimately I think the real killer was the period 1996-2003. Those momentous 7 years contained a one-two-three punch: Clinton got impeached by the House on the unlikeliest of unlikely events, namely physical proof of his lying under oath. Then Bush won in 2000 on the slimmest of margins via "hanging chads" in Florida. (Anecdotally, the 2000 election certainly radicalized Democrats at my workplace like Rick B. in a way I’d not seen before.) Bush didn’t have a chance to begin with given the election chaos, but then doubled down and threw gas on the fire by getting into an unnecessary war in Iraq on faulty pretenses (the phantom WMDs). I don’t think the republic ever recovered from the way Bush won and how he governed. It was "total war" at that point on the Dem side.
The Republicans,conciliatorily, offered meek and mild candidates in the form of a maverick and the Mormon, McCain and Romney, but Obama rammed Obamacare down our throats and crucified social conservatives. Republicans retaliated with the total war approach of Trump. Democrats retailiated withe collusion hoax...,