After three months (and possibly more) of this primary continuing, how much infighting between Clinton and Obama supporters is going to cause them to think McCain is a second choice to their first option?
I think the only safe assumption about that is that the share will be smaller come election time than the polls now show.
That mostly means the difference between Clinton and Obama will likely be smaller than the polls now show, too.
One might infer from my first sentence that McCain is bound to do worse on balance than polls now show, but (while possible) I'm not making that claim; Reps are hardly united behind their candidate yet either, and quite a few of them, too, are currently supporting an unusual second choice (Obama or Clinton) after not getting their first choice (one of the non-McCain Republicans). Many of them are going to come back as well. And there's the whole "Dems currently more fired up because the campaign's still going; this shows up in polls" line of argument that I'm not sure I'm buying but cannot dismiss either.