This book is about the formation and dissolution of an alliance between a social movement and a political party during a time of high partisan polarization. The story of this alliance is not about back-room deals among elites who negotiated a partnership and then eventually betrayed their allies. In contrast to prior research on party-movement alliances, ours is a story about a multitude of participants in a movement and a party who separately shifted their attention and effort among causes in light of developing political events. In telling this story, we emphasize how macrolevel outcomes were a consequence of these individual-level behaviors. The shifts were driven in part by intersectional identities that were critical lenses through which actors interpreted events and decided where to focus their energy. The fact that partisan identities tended to be stronger and more enduring than their intersecting movement identities led the party in the street to serve the interests of the party over the interests of the movement. In the case of the antiwar movement and the Democratic Party after 9/11, this pattern played out through the behavior of grassroots activists, advocacy organizations, and members of Congress.

We document the collapse of the antiwar movement after the election of President Barack Obama. Key to our argument is that this collapse was led by Democratically identified individual activists, organizations, coalitions, legislators, and funders. Of course, Democrats were not the only ones who left the movement. The movement declined for a variety of reasons, such as activist burnout from years of protest and the scarcity of funding after the global financial crisis of 2008–2009. But the fact that these declines occurred disproportionately among Democratically identified sources lends strong support to our partisan identification theory of mobilization. In contrast to prior research on social movement mobilization, ours helps to explain how partisan identifications shaped actors' interpretations of ambiguous U.S. foreign policies, how they allocated energy to antiwar movement goals, and why they shifted attention to other policy issues, such as health care.