I wondered yesterday why so many Hillary Clinton supporters in Indiana and North Carolina said they wouldn’t vote for Barack Obama if he were the nominee. What I really wondered was if race was a factor in that.

Well Jed Lewison has a pretty good explanation of the phenomenon. The one-time aide to Sen. Maria Cantwell pushed himself away from the Vegas poker tables long enough to send me an explanation of the role of the “McCain Meddlers.”

From the exit poll, 1 in 8 Indiana Clinton voters and 1 in 6 North Carolina Indiana voters will not vote for Clinton even if she wins the nomination. These are the “McCain meddlers.”

If you factor them out (also subtracting the McCain meddlers who supported Obama), Obama wins Indiana by 3 and North Carolina by 22.

So what Lewison is saying is that no one should draw too much from the fact that Clinton supporters say they won’t back Obama, because a chunk of them aren’t really Democratic supporters.
And as he writes at The Jed Report:

By the books, there's no taking away Clinton's Indiana victory, but it is worth remember that it was only made possible by voters who have no intention of voting for a Democrat in the fall.

He explains his methodology here.