This is tough,
If Billary gets the nod than I would expect McCain to win by default, just the people alone from both parties that will cast votes against Billary, and then there are those that also support him. If that showdown happens expect to see him win by the largest margin since 1980.

If he is running against Osama then it's a tough call.
Personally I am pretty much voting against whomever the dem is and just leaving it at that. It will be intresting to see who VPs are going to be, and if they will have any impact on the race.