Like I touched on in another recent thread, this morning... Trump's base is going to get "fired up" about whichever democrat runs. Precisely because there's a propaganda machine that's got them so fearful of that their religion is under attack, their country is under attack from outsiders, there's a "global agenda" looking to destroy America, and that the democrats calling for gun control is actually part of some plot to disarm them so that outside forces can come in and control them.
The culmination of these irrational fears is that it allows them to be easily manipulated into believing just about any nonsense conspiracies that far-right outlets claim about any democrat that's running for office.
We saw it with Obama, we saw it with Hillary, and we'll see it again with whoever gets the nod in 2020.
As long as right-wing propaganda machines and exploit the faith & fear of many conservatives (while actually working for a handful of uber-rich, and not them), and present themselves as the knights in shining armor that can protect them from it, we're going to continue to see this pattern.
