Gallup observes that President Obama’s approval rating averaged 43 percent in November, one of the worst showings by a modern president at this point in his term. Jimmy Carter scored worse — with a 40 percent average — though his poll numbers rallied after the Iranian hostage crisis. (By March 1980, his numbers had returned to the low-40s range.)
Generally Gallup finds that presidents win reelection when they have at least 50 percent support at the time of the election, though George W. Bush was an exception (48 percent). Still, such historical analyses as these rely on a limited sample size.