Key Republicans say they won’t back Donald Trump if he’s the GOP nominee. That may make them feel good — and seem principled. But from a practical standpoint, it makes no sense. Not when the next president will choose one or possibly more justices for the Supreme Court. I say this as a conservative who has fought many fights as the GOP’s chief counsel to the Senate Judiciary Committee, including for the judicial nominations of Miguel Estrada, Brett Kavanagh and Priscilla Owens, among others. The idea of the party not coalescing behind the eventual nominee would simply be mad. There’s...