Giuliani/Santorum would probably carry NY, PA, NJ and MI. If they could hold the South and Rocky Mountain states, it'd be a landslide. Giuliani's got three years to reposition himself on social issues -- remember that Bush I was originally pro-choice. Having a strong pro-life candidate on the ticket would also help.
There's nothing Giuliuani can do to prevent millions of Christians from staying home on election day if he's at the top of the ticket. He is who he is: a pro-abort, pro-gay social liberal.
He'd have to build a winning coalition without much of the South, which is impossible. Ergo, if he is nominated, he will lose.