Mr. Trump’s virulent combination of ignorance, emotional instability, demagogy, solipsism and vindictiveness would do more than result in a failed presidency; it could very well lead to national catastrophe. The prospect of Donald Trump as commander in chief should send a chill down the spine of every American.

For Republicans, there is an additional reason not to vote for Mr. Trump. His nomination would pose a profound threat to the Republican Party and conservatism, in ways that Hillary Clinton never could. For while Mrs. Clinton could inflict a defeat on the Republican Party, she could not redefine it. But Mr. Trump, if he were the Republican nominee, would.

Mr. Trump’s presence in the 2016 race has already had pernicious effects, but they’re nothing compared with what would happen if he were the Republican standard-bearer. The nominee, after all, is the leader of the party; he gives it shape and definition. If Mr. Trump heads the Republican Party, it will no longer be a conservative party; it will be an angry, bigoted, populist one. Mr. Trump would represent a dramatic break with and a fundamental assault on the party’s best traditions…

I will go further: Mr. Trump is precisely the kind of man our system of government was designed to avoid, the type of leader our founders feared — a demagogic figure who does not view himself as part of our constitutional system but rather as an alternative to it.