Interest Groups and Legislative Ideology: Who Influences Whom?

What is the relationship between the ideological positions of state legislatures and interest groups in that same state, assuming there is one? Using data on the counts of group populations along with data on the ideological leanings of state legislative majority parties, I find that while variation in group populations does not affect legislative ideology, the ideological positions of legislative majority parties does influence group numbers, though the relationship is nonlinear. Liberal legislatures are particularly influential on the number of groups in a state. It also turns out that legislative party ideology has a conditional effect on the number of cause-oriented advocacy groups and business-oriented associations in a state, mitigating the effect of associations on advocacy groups while enhancing the effect of advocacy groups by business associations. Overall, though, the data show that it is legislative parties that influence group population sizes and compositions, not the other way around.