Fractious conservatives are already on side.
For myself, and I think most other conservatives, that is enough of a reason.
If you wish to argue I am not as purely conservative as you, then fine. Maybe you are. I don't care. My interest is voting for who is better for the country, not in impressing anyone about my credentials as a "true" conservative.
If you want to argue that I will then be responsible for all the lousy decisions McCain will inevitably make in office, then I will reject your criticism unless you show good reason to think Obama or Clinton would have faired better.