Depending on how exactly you mean the wording of the OP, if you accept a 2004 POD that makes Obama Senator but ot president, the key is Obama' keynote in Boston. Either a no performance or poor performance could do in the 2008 run.
This article detailing the Speech gives some good PODs.
For a no performance, it's simple. Jennifer Granholm or another speaker gets it for one of a variety of reasons. (Telling
The Atlantic Monthly, "Sometimes Kerry just doesn't have that oomph." for example.)
A less likely possibility is having the state budget fight go on a few more days.
For a poor performance there are several possibilities:
Obama's a breakfast or lunch can disagree with him. According the above article, breakfast was an omlette and lunch was a turkey sandwich, and either could cause food poisoning fairly easily.
Obama had trouble adjusting his speaking style.
Adding to the tension in Boston was Obama's rough start at rehearsals. ... Obama, who prefers speaking extemporaneously, had no experience working with a teleprompter or addressing a group this loud and lively. ... Obama struggled early on to master the mechanics of this new speaking environment. First, he had to train himself to read the words off the teleprompter screens without having it look or sound as if he were reading. He also had to adjust his speaking style.
If he doesn't get all that down, he'd do poorly.
Also, he was giving so many interviews that he was starting to lose his voice.
Gibbs recalls that by midafternoon, Obama's voice was getting hoarse from overuse. In the middle of one taping, Gibbs had to search the FleetCenter for hot tea and lemons to help soothe Obama's throat. "I can remember having this panic attack," he says: "Oh, my God, what if he's written this tremendous speech and everything's great, except I've scheduled too many TV interviews and he loses his voice?"
Finally, if you watched The Speech, you may remember he had some trouble hitting his stride at the start. If he didn't it wouldn't have been nearly as effective.
So, for a really awful performance, throw those three together, and Obama bombs the Keynote. Add in the tensions with Kerry's "stealing" his best line, and you can put off 2008.
Of the remaining possible 2008 DP candidates, Kerry and Gore remain damaged goods, Clinton has too much baggage from her husband's term, Edwards has the scandal, Richardson had potential scandals, Gravel was going Green, and Kucinich is unliklely to win the natuional race. That leaves Biden, Dodd, Vilsack, Bayh, and probably Mark Warner, and my insinct goes to Bayh.
Meanwhile, Obama sees the writing on the wall in the 2010 and better opportunity back home, doesn't run for re-election, but instead runs for mayor in 2011. Then, after getting some executive experience, he runs for the presidency in either 2016 or 2020.