That's what I've often wondered when reading these crossover explanations for McCain's win in NH. And Obama had won Iowa and NH was a comeback win for Hillary. It's hard to believe that large numbers of Dims skipped that race and voted in the Republican primary.
And McCain did go on to win South Carolina. After that, his wins were mostly in blue states and Florida.
There was a contingent here at Free Republic that were supportive of a conservatives vote for John McCain in NH; they wanted to stop Romney in the belief that McCain could never win anyway, and stopping Romney would clear the way for Fred Thompson.
Whether that movement had any effect or not, I can’t say.