Obama's presidency is certain to disappoint, no matter what he does or doesn't do. The expectations for him are not only unusually high, but contradictory. His liberal supporters are happy to finally get an unabashed liberal in the White House, while his conservative supporters hope he will be a pragmatist. If he compromises on rolling back the controversial policies of the Bush years, he will disappoint many supporters, but if he pushes too hard, he risks alienating many other people. He has already made a range of promises that may be hard to implement in the current financial crisis, which may come to define his presidency regardless of how he handles it.
Does this mean he will be un-elected in 2012? Possibly. But that is less likely to happen than many people realize, no matter how far his star falls. He won by defeating the party in power. So far, our nation has seen fifteen presidents reach the office that way and not die in their first term. Those were (in reverse order) Bush, Clinton, Reagan, Carter, Nixon, Eisenhower, FDR, Wilson, Harrison, Cleveland, Grant, Lincoln, Pierce, Polk, and Jefferson. Of that group, eleven were reelected (though Cleveland lost his first reelection bid due to an Electoral College fluke). The remaining four--Polk, Pierce, Harrison, and Carter--each had unique circumstances stand in their way.
Polk never ran for reelection, due to poor health. Pierce ran but wasn't nominated by his party. Harrison was nominated and went on to lose in the general election, but he hadn't won the popular vote the first time around (due to the aforementioned Electoral College fluke). Carter is the only president in U.S. history to win the popular and electoral vote against the party in power, serve one term, get nominated for reelection, and lose (popularly and electorally)--an indication of how badly his presidency went. Even then, it took a Reagan to defeat him. In the 1980 election, Carter was actually leading in the polls until Reagan gave a strong debate performance a week before Election Day. Astonishing as it may sound, a weaker Republican candidate than Reagan could easily have guaranteed Carter a second term.
Many of Obama's opponents would like to believe he'll be the next Carter. From the evidence of the campaign, it's unlikely. In the 1976 election, Carter managed to shrink a 33-point lead down to a virtual tie, then win in a squeaker. Obama, in contrast, exceeded the expectations, winning in an electoral landslide that seemed like a stretch just months ago. Regardless of the job Obama does as president, he has proven himself to be a far more skillful politician than Carter ever was. If Republicans fail to accept that fact, they will continue to lose power. They might not be where they are today if they didn't already make that mistake.