I've taken 3 anecdotes and 2 polls that support this theory. I don't know how it applies nationally, but polls conducted by Rasmussen are from Pennsylvania and North Carolina. One with an Obama lead and one with a Clinton lead.

Anecdote 1:One of my very close friends whom I graduated with supports Hillary Clinton. I took it as a given that he would vote for the Democratic nominee. We were recently talking over dinner and was shocked when he indicated that he liked McCain and would vote for him if Hillary was not the nominee. I don't think he disliked Obama, just that he didn't think he was ready for the job.

Anecdote 2: My uncle rarely votes. He voted for her in the Primary (NJ).  My aunt was hosting a small get-together at their place. The TV was on and McCain was talking about something (I never pay attention to anything he says)but both he and my aunt commented on how he would make a good president. And Viola again, against Obama they are voting McCain.

Anecdote 3: Another friend's mother, not political at all; she did not vote in any primary but weary of Bush. She likes all of them, from what she's heard. But voting Hillary or McCain. Again, its McCain instead of Obama.

Everyone I've referenced lives in NJ. Maybe this could explain the recent polls showing Hillary leading McCain by double digits and McCain leading Obama by 2 in the Garden State.
http://www.rasmussenreports.com/public_c ontent/politics/election_20082/2008_pres idential_election/new_jersey/new_jersey_ 2008_presidential_election

Poll 1: From the latest Rasmussen Poll from Pa." If Obama does, in fact, win the nomination, just 57% of Clinton voters say they are even somewhat likely to vote for him against John McCain.

If Clinton is the nominee, just 64% of Obama voters say they are at least somewhat likely to vote for her against McCain."

http://www.rasmussenreports.com/public_c ontent/politics/election_20082/2008_pres idential_election/pennsylvania/pennsylva nia_democratic_presidential_primary

Poll 2: From the latest Rasmussen Poll from NC. "If Clinton wins the nomination, just 67% of Obama's voters say they are at least somewhat likely to vote for her in the general election against John McCain.

If Obama wins the nomination, just 49% of Clinton voters say they're Somewhat or Very Likely to vote for Obama against McCain."

http://www.rasmussenreports.com/public_c ontent/politics/election_20082/2008_pres idential_election/north_carolina/north_c arolina_democratic_primary

I think this flies in the face of all the those who are saying "But he can win red states". Winning red states is great, but only if you can hold down the big blue ones first.