Theories of political participation answer the question "Who participates in politics?" but are strangely silent on an equally important question: "What do they say?" This paper investigates the dynamics of activist speech using evidence from a representative panel of nearly 8,000,000 posts from some 6,000 political blogs. Drawing on the full-text content of these posts and an accompanying survey of the bloggers wrote wrote them, we find that that differences in bloggers' motivations have a direct impact on the topics, tone, and timing of their blog posts. These "motivations" include both personal characteristics (e.g. partisanship, political knowledge, ideological extremism) and institutional considerations (e.g. reliance on advertising revenue, network position). We also find evidence against "online exceptionalism," strongly suggesting that lessons learned about online activists can usually be applied to offline activists as well. These findings and methods have implications for theories in many fields related to political communication, especially public opinion, participation, media, campaigns and elections, civil society, and representation.