The best hope for what’s left of a serious conservative movement in America is the election in November of a Democratic president, held in check by a Republican Congress. Conservatives can survive liberal administrations, especially those whose predictable failures lead to healthy restorations—think Carter, then Reagan. What isn’t survivable is a Republican president who is part Know Nothing, part Smoot-Hawley and part John Birch. The stain of a Trump administration would cripple the conservative cause for a generation.

This...