Trump's Republican problem

Long-time readers (both of them) will know of my affection for "my favorite wonk", Megan McArdle. She has been canvassing on Twitter for information about where Donald Trump support is coming from and where it won't ever come from, and has just published a great list of anonymous quotes from lifelong Republicans who won't vote Trump even if he's the Republican candidate:

• I've always voted Republican [...] I have generally avoided voting third-party for fear of helping the Democratic candidate win. However, if Trump wins the nomination, I will vote for the winner of the Libertarian party nomination. I will not support Trump under any circumstances.
• [I will] stay home or not vote for President if Trump is the Republican nominee. After voting basically a straight ticket Republican since I have been eligible to vote, this is truly amazing.
• I have never voted for a Democrat before, but I care too much about the future of this country to let a blithering imbecile become the President.
• I hate Hillary Clinton, but at the very least I know she will do what [she thinks] is best for this country. I cannot say the same about Trump.

I'd say "read the whole thing", but be aware that you'll be there a while.

I really can't imagine many Democrats voting for (say) Republican Ted Cruz if the Democrat candidate was either Sanders or Clinton, no matter what they though of the Democrat. I wonder if this will turn out to be the most compelling reason for the Republicans to band together and stop Trump - not so much to stop him being President, but to stop him being such a horrible Republican candidate that he would keep Republican supporters at home and let in Clinton or Sanders.