California Watch -- the independent journalism collaborative founded by the Center for Investigative Reporting and staffed by top shelf journalists -- has created a database that enables researchers to input California bill numbers and learn which interests lobbied on the measures. This data is drawn from California Form 635 on which registered lobbyists declare what measures they have lobbied, along with the aggregate amount they have spent on lobbying. Prior to the California Watch system going live, researchers could access the data by interest group (i.e., by reviewing each lobbyist's filing) but not by measure number. So, this tool will enable a whole new type of analysis.
Read more about the system and some noted limitations here.