This paper aims to understand how party organization differs across challenger and mainstream parties. In earlier work, we have developed a theory of political innovation in which we view party competition as a constant struggle between mainstream parties, who as dominant market players try to defend their position, and challenger parties, who as disruptive entrepreneurs try to challenge mainstream party dominance through political innovation. In this paper we zoom in on the differences in party organization among challenger and dominant parties to show how this affects their party strategies. By analysing data cross-national and cross-temporal expert and survey data from 14 West European democracies, we highlight the complex relationship between political entrepreneurship and party organization.