I very much want to think that McCain will lose the nomination, and I think a Romney win is plausible. But I have to say that I’m not sure what this data is supposed to prove. McCain’s declining vote share says very little about his chances and a lot more about the banal factthat having 4 serious campaigns in a state plus the unusually well-funded vanity campaigns of Paul and Rudy! tends to depress the vote share of the frontrunners when compared to a campaign with 2 serious candidates. (This would seem to be a variant of the “Bill Clinton never won a majority” argument, as if no Perot voters would have gone to him in ’96.) The argument from raw vote totals is even worse; it is certainly bad news for the Republican Party but says absolutely nothing about McCain’s ability to win future primaries. The latter seems to be a variant of the world-historically specious “Bush got more votes than anyone in American History!!!!What a landslide!!!1ONE11!!11” argument.
Now, if someone has some evidence that supporters of Thompson and Huckabee or Rudy’s supporter will disproportionately vote for Romney over McCain, or at least a compelling logical argument on behalf of this outcome, then we’ll have something. But I haven’t seen either yet.