Obama's not so sure they'll go to Hillary. Here's the relevant bit from an interview he did with the Christian Broadcasting Network's David Brody:
BRODY: Will Hillary be a drag for down-ticket races as a presidential candidate?
OBAMA: I think there is no doubt that she has higher negatives than any of the remaining democratic candidates. That's just a fact and there are some who will not vote for her. If you look at the results in Nevada, for example, she eked out the popular vote victory over me, but I ended up winning more delegates because she got almost all of her votes from Clark County, Las Vegas and some of the traditional democratic areas. We got votes there, but we also got votes in northern Nevada and rural conservative regions of the state that traditionally don't vote Democratic, but were excited about my campaign.
I have no doubt that once the nomination contest is over, I will get the people who voted for her. Now the question is can she get the people who voted for me? And I think that describes sort of one of the choices that people have, just a practical choice, as they move forward." [Emphasis added.]
The Clintons' strategy seems to be banking on the idea that even if they take the low road to winning the nomination, Hillary will still get Obama's old supporters in the general election, because those voters will have nowhere else to go. I don't think there are too many of them who would go to McCain, but what if they just stayed home? It's probably not an electability argument Obama can make too explicitly without sounding like a spoiler--I'm actually surprised he made it as explicitly as he did to Brody--but it's something to think about.