I think it's more than just social media amplifying it. It's pretty clear that people go down rabbit holes and get increasingly radicalized. I'm not sure I'd dismiss it as mental illness.

It's closer to someone who's been sucked into a cult. I do think that online spaces and discourse have warped people's brains where it is a lot easier to get sucked into more extreme ideology and logic and harder to stop the tumble down the rabbit hole.

And, no, this is not a phenomenon limited to one party or political ideology.