My hypothesis, that has held up (in my mind) for awhile now, is that it's an adherence to ideology over reason and rationality, an approach that unfortunately has been legitimized for a large part of the right wing in the U.S. The denial of climate change is perhaps the easiest example, but remember that large proportions of Republicans believe many other things also supported by ideology and contrary to reason: Obama was not American, that Saddam Hussein was involved in 9/11, and many other myths. During the campaign, Trump could say almost anything and a certain segment believed it; and no matter what a Democrat said, it was disbelieved.

They aren't the first to do that, but I can't think of it happening on such a large scale since the Communists and fascists. In a way, it's reminiscent of the struggle that ended the Middle Ages and gave rise to the Enlightenment.