We model the behavior of a vote-maximizing legislator in order to predict interest group campaign contributions to incumbent politicians. We show that committee assignments and voter preferences affect the price a legislator requires to produce policies for any interest group. An econometric analysis of actual interest group contributions shows that these groups make significantly larger contributions to legislators on committees with jurisdiction over especially relevant policy issues and to incumbents with non-hostile constituencies. These results support our theory; interest groups act as if committees matter in the determination of policy and voters' interests constrain interest group behavior.