The decline in traditional methods of civic engagement is a cause for concern in many Western democracies. Similarly, studies of American party politics point to a transformation from locally-based volunteer organizations to national ones assisting candidate-centered, professionally-run campaigns, leaving little room for volunteer participants. This thesis analyses the recent resurgence of grassroots participation and organization in the United States. Using interpretive methods, I present a study of grassroots participants in Massachusetts Democratic Party primary campaigns in 2006. Primary documents, interviews with volunteers and paid members of field staff, and observations of canvassing work all detail the personal and organizational contexts of participation, illuminating the meanings individuals found in campaign work. Grassroots participation takes place in a loosely organized set of candidate-based campaigns, local party committees, and civic spheres. When participants first engage in this environment, they become socialized into a community with learned norms, practices, and ways of knowing. While those interviewed shared some of the motivations of party activists in previous studies, the motives and beliefs described by both professional organizers and volunteers were less policy focused than expected, and blurred the distinction between ideological and social categories. Indeed, while organizers and volunteers build distinct identities through their campaign participation, they share many more similarities than the literature on activism and professionalism in parties would suggest. Participants also serve a crucial role as translators between party elites and their fellow citizens, with important implications for linkage and the problem of decoupling. Rather than a return to traditional methods and structures of political engagement, the participants observed take part in and are building communities which have much in common with new forms of non-traditional participation. These findings contribute to the development of party organization theories and point towards the need for greater dialogue between scholars of party politics, organizational studies, and civic engagement.