A growing body of research suggests that Twitter has become a key resource for networked counterpublics to intervene in popular discourse about racism and policing in the United States. At the same time, claims that online communication necessarily results in polarized echo chambers are common. In response to these seemingly contrary impulses in communication research, we explore how the contested online network comprised of tweets about the April 2015 protests in Baltimore, Maryland, evolved as users constructed meaning and debated questions of protest and race. We find that even within this highly polarized debate, counterpublic frames found widespread support on Twitter. Progressive racial justice messages were advanced, in part, by brokers who worked across polarized subcommunities in the network to build mutual understanding and model effective strategies for reconciling disparate accounts of protest events.