Even Jimmy Carter got his party's nomination. You can't beat somebody with nobody and it doesn't look like there's any Democrat who will challenge him.
If Hillary does, it may change things, but politicians like the patronage: so long as Obama has jobs for them and money to pass around they aren't going to turn on him.
It would be interesting if Russ Feingold challenges Obama from the left, but I don't see him losing the nomination. Democrats invested too much in him to back away easily.
The donors won’t back a loser; they’ll support a (very) white, probably Southern, Christian (at least in name) candidate with no track record. There will be a face-saving exit for Obama, who certainly doesn’t want to be the first Mulatto to lose a presidential election.