What motivates parties to change their positions? Earlier studies demonstrate that parties change their position in response to environmental incentives, such as voter shifts. Yet, this work also suggests that parties differ in their responses. What accounts for this variation? We argue and empirically substantiate that differences in party organization explain the divergent responses of parties to environmental incentives. By means of a pooled time-series analysis of 55 parties in 10 European democracies between 1977 and 2003, this study demonstrates how the party organizational balance-of-power between party activists and party leaders conditions the extent to which environmental incentives (mean voter change, party voter change, and office exclusion) drive party-position change. The study's findings have important implications for our understanding of parties' electoral strategies as well as for models of representation. © 2013 Southern Political Science Association.