As the candidates who will qualify for the presidential debate in August begin their preparations, they are going to want to bone up on a range of policy issues and make sure they have coherent answers to hard questions. (Now that Syria is a mess what do we do about it? If a Supreme Court vacancy opens on Day One, who is at the top of your replacement list? Has the president pardoned too few or too many people?) Equally important will be a game plan for neutralizing Donald Trump. Even before Trump self-destructed on Saturday, we saw candidates like...