American political parties are durable constructions. But they have been broken before by powerful, roiling issues such as immigration and racial prejudice. Many Republicans could not vote for Trump but would have a horribly difficult time voting for Clinton. The humane values of Republicanism would need to find a temporary home, which would necessitate the creation of a third party. This might help elect Clinton, but it would preserve something of conservatism, held in trust, in the hope of better days.
Ultimately, these political matters are quite personal. I have spent 25 years in the company of compassionate conservatives, reform conservatives, Sam’s Club conservatives or whatever they want to call themselves, trying to advance an agenda of social justice in America’s center-right party. We have shared a belief that sound public policy — promoting opportunity, along with the skills and values necessary to grasp it — can improve the lives of our fellow citizens and thus make politics an honorable adventure.
The nomination of Trump would reduce Republican politics — at the presidential level — to an enterprise of squalid prejudice. And many Republicans could not follow, precisely because they are Republicans. By seizing the GOP, Trump would break it to pieces.