The recent academic literature suggests that pressure from special interest groups has little or no influence on whether initiatives and referendums are passed or defeated. Further, there is a consensus that, to the degree that groups' campaigning is important for explaining outcomes, groups opposing the initiative and favoring the status quo have an advantage over groups that support change. These studies have not considered that interest groups campaign strategically and therefore that campaigning is endogenous in ballot measure elections. This study examines the effect of campaigning on ballot proposition elections and develops a research design that accounts for strategic and endogenous campaign advertising. The research design uses a two-way fixed-effects model to estimate the effect of interest group pressure on ballot measure outcomes. The data are based on television advertising for or against California ballot measures from 2000 to 2004. The results show that supporting and opposing interest groups' campaigning has a quantitatively important and statistically significant influence on ballot measure outcomes. The campaigning of supporting interest groups is at least as productive as that of opposing interest groups.