>"Which means that finding a single individual who can coherently represent all of them is basically impossible."
And by the time the Republican nomination was over, the strength of the social conservatives had dragged the rhetoric so far right that it was difficult for Romney to get back to the centre for the election.
These two comments very completely and eloquently sum up the problem. The divide between social and fiscal conservatism is something that gives the republican party the biggest disadvantage.
I should also mention that the big-L Libertarians, as instantiated in the U.S., are not the fiscally-conservative socially-liberal party that centrists would even entertain. They're far too radical to be at all politically viable (abolish Fed and Education, open borders, legalize drugs, etc.).
And by the time the Republican nomination was over, the strength of the social conservatives had dragged the rhetoric so far right that it was difficult for Romney to get back to the centre for the election.