Maybe it's simpler than Nate Silver believes. Money quote:
The level of the unemployment rate alone gives little guidance. The unemployment rate in 1980 was 7.2%, up considerably from the year before; Jimmy Carter duly lost his bid for reelection. In 1984, the unemployment rate was actually higher at 7.5%, but was on its way down from the previous year; Ronald Reagan was reelected in a landslide. In both cases, it was the trend that mattered.
The Dish 