I don't understand what you are saying here. From a pure policy perspective, Clinton and Obama are nearly identical. , a Hillary precinct captain and I were having this very discussion yesterday while we spent the entire day working the polls. They both have fairly progressive agendas. The reason I support Obama over Hillary has more to do with his approach to politics and how he plans on getting his agenda realized. But I can't be honest with myself, as a progressive, and say that I am more aligned with McCain than I am Hillary, because it is simply not true.
I will concede that there is a part of me that would be resentful if Hillary stole the election and I might be tempted to vote for McCain. But if I did, it would be out of spite and not because I find myself more aligned with McCain's views. I suspect that your "significant portion" of Clinton supporters would be voting McCain for the same reason.*
*Unless, of course, you are referring to the Clinton supporters who voted for Clinton at the direction of Limbaugh. I am sure their points of view are more aligned with McCain's than Obama's.