The Incumbent Formula
How hard is it for incumbents to be re-elected? Although there are many factors at stake, we decided to look at just two here: whether the economy was in recession at the time of the election, and whether unemployment had decreased since the last election. Adjust the filters yourself to see how incumbents have fared since 1900.
As you can see, Obama has a 6/7 chance of winning (historically) if he can keep the economy out of recession and unemployment below where it stood at the start of his term. Even under the most dire of circumstances (recession and worsening unemployment), he still has a 50% chance.