Gallup also has a nifty tool showing the polling from all the Presidents since Truman. Comparing Obama to these previous Presidents is somewhat interesting. I think the ones who most closely track Obama right now are Carter, Reagan, and Clinton:
From about day 220 until about day 800 in their respective Presidencies, they all mostly stay within a 10 percentage point range of each other. Carter then starts diverging downward, with the exception of ~90 days right after the Iranian hostage disaster.
In this group, Reagan had the worst middle, dropping down into the 30's, but he began a steady rise as his policies pushed the economy into a sustained expansion. Clinton likewise began a steady rise in his 3rd year, when he started pursuing policies like free trade and welfare reform, and abandoned the "Hillarycare" fiasco.
Obama, while articulating the entirely correct phrase, "Good policy is good politics" has unfortunately instead been a poster child for the opposite: "Terrible policies are terrible politics".
His policies have turned what should have been a strong recovery into a near Depression, and will likely lead to a double dip recession. This has resulted in the current economic stagnation that shows no sign of ending until his term is over and someone else can begin undoing the harm he and (to a far lesser extent) Bush have caused.
This is why you can see that at this point in their administrations, Reagan and Clinton had already begun the upward trajectories that would carry them into a second term - they were respectively 5 and 6 points ahead of Obama, who's own trajectory is instead pointing downward with no reasonable expectation of relief in sight.
Obamacare still has only begun to show its malignant effects on the economy; the regulations Obama is STILL promulgating are continuing to strangle any hope of recovery; Obama threatens new taxes every chance he gets; the trillion plus dollars of deficit red ink continues to flow. Major scandals have erupted, and the media-enabled coverups (only now starting to crumble) will only serve to make them just as, if not more, relevant this time next year when the election season is at its most intense and voters are paying the most attention. There are additional scandals on the horizon too, but Fast and Furious and Solyndra alone are more than sufficient to hurt Obama significantly when he likely wouldn't even be able to survive the damage his disastrous economy has done to his re-election chances.