My latest ZohoSheet project. This will give you a simple popular vote projection for November based on (i) a candidate's support from each party and (ii) turnout rates.
A couple of notes: the party ID values you see here are based on a combination of three recent surveys: Gallup, Rasmussen , and Pew. All three show the Democrats with between a 9 and 10 point partisan edge on the Republicans, although the surveys differ somewhat in how many independents they identify.
I recommend that you not play with the party ID numbers, since those numbers move glacially and are at least somewhat exogenous to the political contest in any given year. Instead, you can manipulate the turnout rates. I have Democrats and Republicans each turning out at 62 percent, with independent turnout being slightly lower. I believe this is roughly what the turnout rates looked like in 2004, although I can't find any hard evidence on that.
The other numbers you see in the worksheet, while not coming from anywhere in particular, are not entirely arbitrary either. Fundamentally, it is very challenging for McCain to be working from this deficit in partisan ID.