Yeah, there have been militia weirdos and racist hate groups for few decades.
But they have mainly been in the rural areas, favoring Idaho and surroundings and formerly with outposts in San Diego.
The guys who blew up the OKC Fed building were somewhat connected with that.
The GOP has also been denigrating the Fed Gov't for a long time now.
(they love that Constitution though!)
Trump as an outsider, cult figure, who rails against gov't (the Deep State!), supports conspiracy theories, and has contempt for the media could be the type of demagogue to catalyze all these disaffected armed losers into a political mob. We've never had such a lone wolf, demagogue, unbeholden to a party establishment as Trump before. We're probably fortunate Trump isn't organized or disciplined enough to pull it off.
One problem is that the USA is such a big country. It's not like in Europe where all you really need to do is intimidate and trample folks in the capital city and your work is largely done. And as I said, the cities in the US are liberal bastions.