Most political parties that emerged in Europe since 1945 have roots in social movements, but unlike older mainstream parties with movement origins they did not adopt an electoral-professional party model focused on pursuing the median voter. Neither have they retained their organizational connections to movements, thus foregoing both the advantages of a "catch-all" platform and access to the mobilizing capacity of movement organizations. Why? I employ a multi-method design combining the analysis of voters and movement party success with original data from fieldwork in Sweden and Germany on four movements and the Green and Pirate parties they spawned. I find that movements are incentivized to be organizationally distinct from parties in order to maximize their public support and influence on government policy. Parties cannot, however, break with their movement origins programmatically, because they exist in saturated party systems and rely on issue voters who are disappointed with the dominant mainstream party models. In that environment social movement organizations are empowered vis-à-vis the parties they spawned and able to influence parties' platforms through four distinct mechanisms. In that way social movements, together with the parties they spawn, counterbalance the trend towards catch-all and cartel parties and thereby (re-)integrate disaffected citizens into the democratic process.