The night of the Indiana primary, I was musing here about exit polls showing that a significant number of Hillary Clinton voters plan to vote for John McCain in November even if Clinton is the nominee. The Jed Report -- a statistically-sharp pro-Obama blog -- breaks this down further. His conclusion:
Therefore, among voters who are not committed McCain supporters, Barack Obama won a 3.6 point victory, a swing of nearly 5 points from the current vote tally.
Basically, it seems beyond dispute that, were it not for crossover voters supporting candidates they don't intend to support in the fall, Obama would have won Indiana. (Jed crunches the numbers and figures the margin would have been 3.6%.) I don't think this is totally unprecedented -- lots of mischievous Democrats crossed over to support McCain in the 2000 Michigan primary, and for all I know they could have provided the winning margin -- but, given that Clinton would almost certainly have dropped out if she lost Indiana, this is quite significant.