What about if the economy totally tanks and we have another major depression under his watch?
We already know the economy is going to totally tank and we'll have a major depression. Biggest since the thirties. That's been clear for a couple of months now and is getting ever clearer, just look at what all the economists are saying.
So yeah, although it's sure to happen "on Obama's watch", it'll be a little hard to blame him for it (though I have no doubt they will try).
What counts now is how successfully he can at least mitigate the effects and reduce the length of the depression.
Of course, there'll be no objective way to measure his success, because it all depends on how large you estimate the depression would become without the actions he will take against it.
So basically, conservatives will bitch that he's only made it worse, and progressives will argue that it would have been much worse otherwise.