Well, almost nothing.
Also, Andrew Sullivan highlighted this bit of the piece first:
Defeating an incumbent president, historically, seems to require either a major scandal, a failed war, or a terrible economy. We have a terrible economy. But the direction seems to matter more than the level — Ronald Reagan famously cruised to reelection with a high unemployment rate because the economy bounced back from the deep but quick 1982 recession. Mitt Romney has made the state of the economy his central theme against Obama. The entire premise of his campaign is that the economy is bad because Obama's economic program has failed.
If voters think the economy is improving, Romney has no ammunition left. That is still the smart play for Romney, because if the economy feels strong, he probably can't win anyway, so he needs to plan for the scenario that gives him a chance to win. A few months ago, that scenario was looking almost certain. Now it's looking far less likely.