I think that's a pretty good analysis. You could be right on McCain drawing more from the right but I don't think so. The McCain attraction has always been the RINO right and those Democrats who cannot stand Hillary and what the Democrat party has become.
Dissatisfaction with President Bush from the right seems to more from people who don't think he's doing enough, rarely that what he's doing is bad. (With the possible exception of the current understanding of the immigration issue. But here, Bush is aligned more to the RINO's than the right. Also those who are adamant over the prescription drug issue or his signing campaign finance reform, etc. All these things put a dent in support but none of them seem major enough to create the backlash necessary.)
If the immigration issue is not handled quickly, both parties are in dangerous territory. The Democrats lost to Abraham Lincoln on the slavery issue and it created the Republican Party; and they suffered with black voting rights issues in the South, so they should be more afraid of this than Republicans, IMHO.
If it goes three ways and it's McCain as an I, Hillary as a D and Rudy as an R, I would bet on Rudy.