Brownback and Santorum have their own "special" wing of the Republican Party. Not quite in the Ashcroft/Moore wing, and nowehere near the Robertson/Falwell wing, but also not in the Bush/Frist wing.
The level of theocracy they want to implement. Rev. Robertson and Falwell want a full theocracy with Christianity as the state religion, Aschcroft and Moore want a mild theocracy, maintaining much of the constitution, but passing laws that cover moral issues, Brownback and Santorum are just right wingers who believe in minor restrictions in personal rights.
