The professionalization of politics and the disappearance of party organizations based on activists seems an inescapable trend. This chapter shows the relevance of organizational rules for explaining the reproduction of party activism. Using data from both an online survey of people differing in their levels of engagement with the FA and in-depth interviews with party activists, we show that those with relatively low levels of engagement – “adherents” – and activists differ in their willingness to cooperate with the party and in the amount of time they devote to party activities. Also, we find that reducing the perceived efficacy of political engagement strongly decreases activists’ self-reported willingness to engage with the party, while this reduction has no effect upon adherents. These findings suggest that the design of organizational rules that grant a political role to grassroots organizers can promote party activism.