He’ll gain a lot of votes from illegal immigrants.
The encouraging news is there are many who voted for Obama who will either now vote for Romney or not at all.
I seriously doubt there is one McCain voter who will vote for Obama, and a much smaller ratio that will not vote at all.
Under this scenario, Romney can easily win. The difference between simply winning, and a 48 state blowout is enthusiasm, and a very large part of that depends on who he picks for VP, and if he does an aggressive campaign after the convention.