Incumbent Party’s Expected Vote Margin = 1.14 −.83 × (Years in Office) +4.51 × (4th-Year Income Growth) +1.66 × (3rd-Year Income Growth) −1.04 × (2nd-Year Income Growth) −2.34 × (1st-Year Income Growth)

This formula tells us how likely it is that a president (any president, not just Obama) is likely to be re-elected. The key factor here is "real disposable income per capita."