This research examines the factors that shape high school students' experiences with an action civics program--"Project Soapbox"--that fosters democratic and social-emotional learning. Drawing on pre- and postsurveys with 204 students, classroom observations, teacher interviews, student work samples, and student focus group interviews, the study illuminates how specific features of the curriculum and its implementation are linked to its promising outcomes. Our findings indicate that the curriculum's emphases and structure, along with instructional decisions and context, play key roles in influencing student outcomes. "Project Soapbox"'s power lies in its alignment with many well-established civic education best practices and in its intentional linkage with key social-emotional learning practices, many of which are newly recognized as having particular civic import.