They are totally personally invested in the idea that their support for Trump is not a mistake. Calling them dumb, which I have been too prone to doing, only further deepens their investment. They are the embodiment of the extremely wise adage that it is far easier to dupe someone than to convince them that they have been duped.
Because his opponent was the “evil” Hillary Clinton, and replaced the “un-American” Barack Obama (he violated campaign finance laws too, you know!!), they have created in their minds this fantasy that it is literally impossible for ANYTHING Trump does to be worse than what would happen if she had won (an illogical concept which I, despite being an ardent Clinton critic, have long fought against). As the ultimate catch-all, this automatically absolves Trump of significant blame for anything because, in their minds, things would be SO much worse if he hadn’t, with the possibly help of God himself, beaten back the beast who threatened to end America. In short, collusion with Russia is good!