The polls which show Obama and Clinton equally electable against McCain are wrong. They reflect the psychology of the electorate today, not in November.
I believe there are 2 great motivating factors for Democrats right now. A passion for Obama, and hatred of George Bush. I know, Hillary has her supporters, but outside of Bill and Chelsea, one would be hard pressed to call any of them passionate. Obama supporters love Obama. Clinton supporters think Hillary would do a good job. The disparity in motivation is substantial and significant.
In November, one of those motivating factors will be gone. The Democratic nominee will not be running against George Bush (as both Dems implicitly are on the nomination campaign trail). We will lose some of the anybody but Bush motivation when it becomes clear that (presumably) McCain too, is (at least marginally) better than Bush.
If Hillary is the nominee, we lose the second motivating factor. |When we combine that with the enormous disappointment that passionate Obama supporters will feel, I expect 2 phenomena will emerge. Many dissapointed Obama supporters will simply feel that if they don't get to vote for Barack, they won'tt want to vote at all. Some Obama supporters will go further and do the election year arithmetic, if McCain wins, they can vote for Obama again in 4 years. If Hillary wins, whether she's elected to a 2nd term or not, it will be 8 years before Obama can run again. Barack was right when he said that if he won, Hillary's supporters would certainly coalesce behind him, but if Hillary won, his supporters would not be as agreeable.
Now combine this with the anti-Hillary Republican vote, and it's quite easy to see how McCain is in a much stronger position against Clinton in November than the polls suggest. For more, see <a href="http://famoustrader.typepad.com/famous_trader/2008/01/reasons-to-vote.html">The Reason to vote for Obama</a>.