National party breakthrough has often been attributed to new or previously minor parties seizing favorable political opportunities. The role of their strategic choices in response to political opportunities, however, has been underexplored because less attention has been paid to relevant negative cases, that is instances when parties encounter favorable conditions without breaking through. This article argues for a historical perspective when selecting these cases and investigates an often overlooked case from Germany’s early postwar democracy: Gustav Heinemann’s All-German People’s Party (GVP). Relying on archival data and historical research, this article reconstructs the conflicts between Catholics and Protestants that provided Heinemann with initially favorable conditions for party breakthrough. Strategic decisions on coalition building, the timing of party formation, and organization building explain the GVP’s failure to seize the opportunity. These findings highlight the importance of case selection as a part of the “historical turn” in political science and of new parties’ agency when explaining (the lack of) party breakthrough. The implications of these findings for the literature on new parties, case selection, and party system development are discussed.