They're choosing a candidate who appeals to one pretty extreme side of people who are already committed Republicans. He won't attract the floating voters they need to win and will even alienate some previously Republican voters.
I think the exact opposite is true. It is the floating voters he is capturing and committed republicans are having a heart attack at the prospect of a Trump as candidate - mostly because they do not believe is a real conservative. He isn't - I actually don't think he knows what it means actually. What has been happening over the past number of years is that the Republicans in their tasteless pandering to the evangelical christian right wing part of the party, have alienated everyone else, proved they are absolutely out of step with the majority of americans, and created this lacuna into which, god help us, Trump stepped.
Mitt Romney is about to make a speech about the situation this morning, proving again, as if it wasn't clear enough, that the Republican establishment understand absolutely nothing about what is really going on here and seem to think if they keep saying "but what you really want is XYX" the electorate will eventually agree with them and vote as instructed. Poor Romney, a decent if arrogant man, believes in himself so strongly that despite 2 defeats he cannot grasp that he has been rejected.
I can't say I'd look forward to a President Trump and hope that Clinton will win, but frankly, he is better than that evil toad, Cruz or that creeping jesus, Rubio.
We already (barely) survived Bush/Chaney and you survived Blair - Trump can't be that bad.