How do social movements intersect with the agendas of mainstream political parties? When they are integrated with parties, are they coopted? Or are they more radically transformative? Examining major episodes of contention in American politics – from the Civil War era to the women's rights and civil rights movements to the Tea Party and Trumpism today – Sidney Tarrow tackles these questions and provides a new account of how the interactions between movements and parties have been transformed over the course of American history. He shows that the relationships between movements and parties have been central to American democratization – at times expanding it and at times threatening its future. Today, movement politics have become more widespread as the parties have become weaker. The future of American democracy hangs in the balance.