crunches the numbers and concludes:

I do not think one can argue that McCain’s wins have been dependent on a divided field. Independents, moderates, and Bush disapprovers have certainly formed the core of McCain’s voting coalition. However, McCain has done what most winning candidates do: win his base by large margins while stealing plenty of voters from the other guy’s base. McCain does not win conservatives or Bush supporters outright – but he has done well enough with them that he could probably win New Hampshire, South Carolina, or Florida in a head-to-head match up.