I don't think he is criticizing Christians for voting for a non-Christian necessarily. Trump aside, statistically, the more Christian a person is the more likely they are to identify as a Republican. With a candidate like Trump, you might think that maybe Christians would have a little bit problem in the moral sense. It's not simply that he is a non Christian. What I am about to say, might be offensive, IDK, I sometimes question if, as a whole, American Christians are following the core values of the faith more or less than they are following the modern political Christian movement of the Republican Party. I do think that political parties are like religion onto themselves for both the left and right. When I find a person who 100% agrees with either party, I tend to think that they aren't thinking for themselves. When I see Christian 100% aligned with the GOP, I wonder if the person is actually a Republican over being a Christian.
So I think the OP is actually trying to get at this question; would the majority Christians ever have have a moral dilemma voting GOP, or if they are just following a political party some reason, maybe a simple reason like the party platform is that abortion is wrong?
Some of us a having trouble understanding the Christian perspective on this.