From an interview with Obama in The New Yorker Magazine November 2006 posted on the leftcoaster.com by eriposte:
"[Q] Where do you find yourself having the biggest differences with Hillary Clinton, politically?
[OBAMA] You know, I think very highly of Hillary. The more I get to know her, the more I admire her. I think she’s the most disciplined—one of the most disciplined people—I’ve ever met. She’s one of the toughest. She’s got an extraordinary intelligence. And she is, she’s somebody who’s in this stuff for the right reasons. She’s passionate about moving the country forward on issues like health care and children. So it’s not clear to me what differences we’ve had since I’ve been in the Senate. I think what people might point to is our different assessments of the war in Iraq, although I’m always careful to say that I was not in the Senate, so perhaps the reason I thought it was such a bad idea was that I didn’t have the benefit of U.S. intelligence. And, for those who did, it might have led to a different set of choices. So that might be something that sort of is obvious. But, again, we were in different circumstances at that time: I was running for the U.S. Senate, she had to take a vote, and casting votes is always a difficult test.
[Q] How would one run against Hillary Clinton, in that sense?
[OBAMA] Oh, I don’t know.
[Q] You never gave it any thought?
[OBAMA] I haven’t.
[Q] You sure?