Party position on public policy are generally a combination of the interest group basis for the party, ideology, and politician self-interest. Maybe some other stuff, too. But certainly, interest groups matter, and many groups are not particularly interested in whether their preferences sort well with the implications of the party's ideological statements. This can, of course, lead to situations in which a party strongly supports a policy which would seem, on the surface, to be very much opposed to their ideology. That's not a bad thing in my view, but it leads to a lot of frustration among the more ideologically inclined party members (to the extent to which many ideologues will reject party identification, because the party isn't "really" conservative -- or, for the Dems, liberal).
Which interest group in the Republican party coalition do you think is most responsible for preventing the GOP from being a true conservative party?