2. Obama has history on his side.
Since 1980, only one U.S. president has failed to gain a second term. That was George H.W. Bush, who defied the odds by succeeding a two-term president of the same party. Since FDR, this has happened only once. It's a tough hill to climb. Americans generally tire of too much single-party dominance. Indeed, that's why Hillary Clinton should take a very hard look at her chances in 2016 -- should Obama be reelected.
A set of three presidents -- Clinton, Bush 43, and perhaps Obama -- is hardly a valid statistical sample, but it does tell you something about the power of the incumbent. It's hard to defeat a sitting president. Although a bad economy offsets some of the incumbent's advantage, Americans tend to get comfortable with their presidents. Presidents are also able to act presidential right up to Election Day. The presidency has a great many bells and whistles, including the White House, which Aaron Sorkin's West Wing president once described as the world's greatest home-court advantage.
There's also the issue of continuity. These days, U.S. state and congressional politics have gotten pretty combustible and polarized. The media circus at the national level only makes things seem more out of control.
As Americans watch their politics implode, they seem to be seeking a measure of stability in the one institution that they all have responsibility for shaping -- the presidency. In these turbulent times, Americans tend to stay with their guys, flawed as those guys may be. Should Obama be reelected, it will only be the second time in U.S. history that America has had three two-term presidents in a row. The last time? Jefferson, Madison, and Monroe. "Throw the bums out" doesn't seem to be as compelling a line these days.