Why are Republicans Republicans?
[T]he stubborn popularity of Mr. Trump, who defies Republican orthodoxy on issue after issue, shows how deeply the party’s elites misjudged the faithfulness of rank-and-file Republicans to conservatism as defined in Washington think tanks and by the party’s elected leaders.
Because politicians and pundits alike are so mired in policy, they overestimate its importance. The default viewpoint, that people have certain policy beliefs, and then they choose the political party that most closely represents those believes, is wrong. It works the other way around. For whatever reason they choose a party, they then come to believe what they are supposed to believe as good loyal party members. The GOPe has gotten its party members to believe some pretty dumb stuff, like for example that tax breaks for the top 1% somehow benefits middle-income Republican voters. (But it trickles down!)