This is not difficult, and it's rather surprising that nobody is saying it: Obama had one major opening to join the primary, and that was that Hillary Clinton, John Edwards, and everybody else (besides Kucinich) voted for the war. Aside from fairly minor differences in health care plans, that was really the only thing he could hold over Clinton and Edwards as a reason why he'd make a better president, and certainly the only position on which he could base an insurgent presidential campaign.
Nor are people exactly split on the war. As of the most recent poll, the number of Americans who believe that going to war with Iraq was "the wrong thing to do" stands at a whopping 62%, almost double the number of people who answered that it was the right thing. As Matthew Yglesias put it:
"...it's inconceivable to me that Obama's campaign could have gotten off the ground had Clinton spent 2002 and 2003 as a lonely liberal voice speaking out against the war, then spent 2005 and 2006 being completely vindicated in her judgment. It's not just that Obama wouldn't have beaten her, he wouldn't have run at all -- it would have been preposterous. She would have faced a from-the-right challenge in the primary that would have gotten some attention but never posed any real threat.
But Clinton's error on the war opened up serious doubts about her substantive and political judgment about one of the highest-profile issues of the moment. In many ways it's a testament to how brilliant her campaign was all throughout 2007 and 2008 that they never allowed the war issue to bury her, considering that an overwhelming majority of Democratic primary voters think she made a mistake. "
The war vote as Achilles' Heel also has irresistible narrative allure. I think there's very little doubt in most people's minds that Hillary Clinton voted to authorize war with Iraq not because she honestly believed it was the right thing to do, but because she believed she would have to if she ever wanted to be president. She's hardly alone in this; it's widely suspected that both John Kerry and John Edwards believed, and did, likewise.
And yet that one vote foiled the presidential ambitions of all three.