I don't think people have gotten stupider or leaders have gotten worse at justifying their policies. IMO what is happening here is a consequence of a catastrophic loss of trust in elites and institutions.
Strangely enough, though, a billionaire slumlord from New York is never what people mean when they say 'elites'.
What's happening is that the Republicans have captured populist messaging pretty well, the Democrats just haven't.
Populism is usually stupid but it's extremely effective if you can flood information stream, particularly new ones. I mean, Radio pretty much made Hitler.
The timing here is perfect. Internet has just become mainstream and is still trusted and our brains haven't developed patterns of discernment. In addition, the economy for every day working people is bad. Its the perfect breeding ground for populism.