When for­mer Pres­i­dent Barack Oba­ma spoke Wednes­day night at the Demo­c­ra­t­ic Nation­al Con­ven­tion, it was strik­ing, and telling, to hear which vot­ers he was try­ing to reach: his vot­ers — the vot­ers he thinks can car­ry his for­mer run­ning mate, Joe Biden, to victory.