Unlike the fear of Islam, which is a rational response to Islamist violence across the world, the fear of Donald Trump really is a phobia. There is a precedent for this: the panicked Reaganphobia that preceded the 1980 election. We heard that Ronald Reagan was a member of the John Birch Society—whose essential creed was “Better Dead Than Red.” He therefore rejected “mutual assured destruction,” the bedrock strategy of the liberal consensus to guarantee coexistence by nuclear deterrence. Reagan, it was said, believed in “counterforce,” that is in a disarming first strike to win a nuclear war.

Mr....