Rebecca Traister has a terrific piece about counterfactuals and the 2008 primary. Amanda has some comments as well. Traister does a good job of outlining the where there may have differences — Obama’s coatails (which were likely decisive in the Hagan and Franken Senate races) versus the unlikelihood that Clinton would have played the debt ceiling hand as badly, for example. But the revisionism that has turned someone with an extensive history of centrist deal-cutting into the second coming of Eugene Debs notwithstanding, the differences would be marginal. (And there’s no doubt that had Obama lost the primary, his supporters would be imagining a left-wing Obama presidency that was never going to happen too.)
Particularly after Obama named Clinton his Secretary of State and adopted Clinton’s signature domestic issue in essentially the form that she advocated it — narrowing the nickel’s worth of difference between them to a penny — in policy terms the 2008 Democratic primary was about almost nothing. For reasons Traister’s excellent book explains, the primary was one of major political and cultural significance — and I don’t want to use the word “symbolic,” which trivializes the very real importance of a primary battle between strong candidates from historically underrepresented and marginalized groups — but not policy significance.
…I agree with a commenter that this is also a good point.