People have already started pondering who will run for president in 2016. And why not? It’s a great parlor game. But any game needs rules; and in this case, history has provided us with a lot ton of precedent to determine who actually has a legitimate shot at becoming the next commander-in-chief, and who is just a much-ballyhooed pretender. So what are those rules?

Rule #1: You don’t become president if you’ve completed a full term in the senate. Only one man has ever done it: LBJ, and he had...