This article introduces a group of politically charged Twitter users that deviates from elite and ordinary users. After mining 20 M tweets related to nearly 200 instances of political protest from 2009 to 2013, we identified a network of individuals tweeting across geographically distant protest hashtags and revisited the term "serial activists." We contacted 191 individuals and conducted 21 in-depth, semi-structured interviews thematically coded to provide a typology of serial activists and their struggles with institutionalized power. We found that these users have an ordinary following, but bridge disparate language communities and facilitate collective action by virtue of their dedication to multiple causes. Serial activists differ from influentials or traditional grassroots activists and their activity challenges Twitter scholarship foregrounding the two-step flow model of communication. The results add a much needed depth to the prevalent data-driven treatment of political Twitter by describing a class of extraordinarily prolific users beyond influentials and the twittertariat.