What we are learning and relearning about the VP pick

Former vice president Joe Biden might not pick his running mate until next week. While that might frustrate many in the media and in the Democratic Party, it is better to move deliberately than to make a blunder. That means letting some trial balloons pop. Over the past week or so, we have learned or relearned several basics about vice-presidential picking.

First, it is virtually impossible to discern if a contender is “rising” or “falling” or whether it is all spin from the contenders’ fans or detractors. The presidential candidate...