Hell hath no fury like a disappointed customer. That’s the central “rebranding” challenge facing Barack Obama as he goes into the final campaign sprint, beginning with this week’s Democratic National Convention. In politics, the central question you face in devising your campaign strategy is whether you’re running as a challenger (a role Obama played to perfection in 2008, promising “hope” and “change”) or an incumbent, which is a marvelous position if the country is thriving, and a terrible one if it isn’t.