Most lobbying in the United States comes from business interests, but not all. Previous work has not paid sufficient attention to how non-business lobbying affects legislative behavior. Firms are more interested in particular goods than advocacy groups which pursue broad-based policy change. These citizen-based organizations often employ grassroots tactics and align with one of the major parties. Advocacy groups are also less likely to support maintaining the status quo. This dissertation argues that interest group lobbyists perform two functions. First, these groups set the agenda by engaging in positive promotion of legislation. Second, advocacy organizations push legislators to vote along party lines in roll-call voting. Using original data on lobbying registrations, bill introductions and roll-call records, I test this argument in Congress and the 50 state legislatures. Advocacy organization lobbying is increasingly prevalent, and the results help explain high levels of party polarization in Congress, and an uneven pattern of polarization in the American state legislatures.