I don’t really believe the preponderance of such people was present before sometime in Obama’s 2nd term, although my “evidence” is sparse and anecdotal.
I’m a bit older than you. My sense is that the Bush/Iraq clusterfuck was a turning point. Howard Dean made, IIRC, a very obvious shift in the rhetoric at time, moving from “Bush’s policies are stupid” to “Bush is stupid”. By his actions, he “authorized” ad hominem attacks on the motives and reasoning ability of his political opponents.
It then amped up again with Birtherism. From that point on, each party seems to assume that the other party is not acting in the best interests of the USA.
Yes, there was tribalism, but IMO it wasn’t nearly universal, as it seems to be today.
The pundits, I think, went tribal during Iraq. That was also the beginning of the ideological attacks on Fox News, if you recall; prior to that, they were a successful but unremarkable right-center news station. It took time to “teach” the citizens to go tribal, and the best students were the people who really WERE students during all this: the millenials.
So, in a way, we’re both right, perhaps. It began during Iraq, and manifested itself fully a decade later.
It’s an interesting question, though. The 2003 rush to war against Iraq, and the number of “left-wing” MSM organs that joined it, was something of a surprise to me.
Funny how all those anti-war groups sort of disappeared during the Obama period, although he managed the war very much in the same way that Bush would have. Plus the Drone War.