It happened post-Obama. Certain groups needing to accept that change was happening and they wouldn't always have 'one of their own' in the highest office in the country, regardless of what party they supported, threw a lot of people off. Made them feel out of control for the first time. Compounding that with a societal shift in people normally marginalized getting a bit of attention, and then young black and brown kids suddenly feeling galvanized as they realized the old rigidity of politics could be shaken up... it was 'too much, too fast' for some of these fukks, and the push-back, even among people who you woulda thought were 'good people' was... well. It was Trump. The party became the bastion for the fiscally obsessed, and the red pill incel crowd who felt like they weren't getting enough love simply for existing and lacking melanin, because that was all a lot of them had going for them to make them feel like they weren't total failures. At least they were white. Obama made them feel insecure; Trump showed up to coddle them; as one of them.
The violent polarizing of the country's political system started almost the day Trump claimed Obama wasn't born in America with literally no evidence, and the media ran the story ad nauseum to the point of Barack needing to give it time and energy to dispute. It showed them that they could come up with dumb shyt and literally sandbag the other party by pushing them to dispute dumb shyt. And years later; the birther group became Trump's base. Because there are people who will believe the worst about non-white people if given any potential reason, no matter how ridiculous. And now four years after that, those people are still his base.