According to this expert, that president's big advantage is that he's not attracted primary opposition in his bid for re-election. The left may be dissatisfied with him for not showiing more economic leadership, being a bit of waffler on same-sex marriage, and letting our troops stay in Afghanistan, but not enough actually to run someone against him.
A glance at recent history, the expert observes, shows that incumbents who don't generate primary opposition win, and those who do lose. In the latter category most recently, there's Ford, Carter, and Bush the elder. (And after all the primary-unchallenged Bush the younger was re-elected, which amazed me at the time.)
In the case of Ford, the way he became president (an unelected VP coming to office because the president resigned rather than be impeached) gave him less legitimacy than any other incumbent in our history. And he seemed to possess the qualities of an able but uninspirational caretaker of the office in troubled times more than those of someone who would want or deserve his own term. So it's really quite amazing that Ford survived the Reagan challenge for the nomination (and that was partly because the Gipper got off to a kind of lazy start). It's just as amazing that he came from way back to almost defeat Carter (showing what a weak candidate Jimmy was). So nobody should generalize from the case of 1976.
In the cases of Bush and Carter, the primary challenges were't the causes of their defeat, although their unusual weakness of their presidencies probably generated the challenges. Republicans were dissatisfied with Bush for lacking his own agenda and raising taxes; he tried to be the heir to the Reagan revolution without any revolutionary fervor. Being merely conservative is no way to maintain your power (see Machiavelli). Buchanan was not a big deal of a challenge, but Bush's shortcomings generated the independent challenge of Perot, which really did do him in. Republicans didn't much care that Bush lost; it was better, many thought, to give up power rather than limp along exhausted and demoralized. (They had been in too long.) (Many Republicans had a similar view in 2008--given the incompetence of of Bush the younger's administration, it might be better to retreat and come back later.)
Carter had alienated himself from all sorts of Democrats, and the country was in, as he said, a messy malaise. It's amazing that he survived the Kennedy challenge, and the victory actually propped him up for a while. (Kennedy lost because of his ambivalence about becoming president; he wasn't driven by the relevant ambition.) The primary challenge was hardly the cause of one-term Jimmy.
Obama, from the beginning, knew that he would have no real trouble from the left. A Progressive president in this day and age is a precious piece of luck that no one with any responsibility on the left would dream of undermining. The Democrats are hardly suffering from fatigue, and, despite it all, they know their president is an admirable man. And of course a challenge from the left to our first African-American president that was the cause of his defeat would be a rather justifiable cause of Democratic regret and blame.
Obama's low approval rating (low to mid 40s) and bad economic numbers do call to mind the case of Carter. Someone might say his situation is closer to Reagan's in 1983, and both the economy and the president bounced backed nicely by November 1984. But we're not going to have that kind of bounce on either front this time. It's likely that the president, if he wins re-election, will do so narrowly, because he hasn't attracted any voters he didn't get the first time and he's certainly lost some, maybe a lot. It's certainly possible that the streak of re-election being connected with no primary opposition will be broken.
To be fair, Obama does have this in common with Reagan: Nobody doubts that he's a real Progressive, just as nobody doubted that Reagan was a real conservative. That's the main reason why neither generated a challenger from within his own party when times got tough.
So it's true that not having any kind of effective challenge from the left gives the president lots of advantages, including ideological and tactical flexibility. A second advantage, of course, is the Republicans haven't come up with a really attractive alternative--no one with the charm and competence of Clinton or Reagan--yet.