This chart by Philip Klein has some interesting information of GDP growth for the March of presidential reelection years.  But the biggest takeaway is perhaps that, as of March 2012, Obama is heading into a general election with the worst unemployment rate of any incumbent since World War II.  Only one president won reelection with an unemployment rate above 6% in the March of an election year: Ronald Reagan in 1984 (when unemployment was 7.8%).  Currently, unemployment is (at least) 8.2%.