Everybody knows that the very first rule in picking a vice president is to do no harm. Really, you can choose anybody. How dumb would you feel if it turned into an embarrassment? This is why the careful, modern candidate sets up a screening system. Now ask yourself: how dumb would you feel if you got in trouble over your selection for the vice-presidential screener. Barack Obama is having the first postprimary crisis, a moment in which the only conceivable response is: what was he thinking? James Johnson, who resigned Wednesday as the leader of the Obama vice-presidential selection committee,...