This dissertation systematically evaluates the causes and consequences of teacher political activism in the postwar United States. It begins by investigating a puzzle that has had important and enduring implications for both American K-12 education and U.S. electoral politics more generally: the transformation of rank-and-file teachers into an active and powerful political constituency organized by influential teachers union interest groups. I first demonstrate that the wave of public sector collective bargaining laws enacted by states during the 1960s and 1970s helped catalyze teacher political activism by decreasing the costs for teachers unions to mobilize rank-and-file teachers to participate more in politics. Exploiting variation in the timing of these law adoptions across the American states, I find that the political activity of all teachers, not simply union members, significantly increased in elections held directly after the introduction of collective bargaining. Substantively, I estimate the effect of a teacher working under a mandatory bargaining law environment as greater than the effect of a teacher's household income, education level, and union membership status on her likelihood of participating in politics. In the second half of the dissertation, I address a separate empirical puzzle that has perplexed scholars for quite some time. Namely, if teachers and their unions are as dominant a force in education politics as the conventional wisdom portends they are, why is the quantitative research literature on teacher unions effects so inconclusive at establishing those effects? In contrast to existing scholarship, I demonstrate that previous efforts to measure whether and how teachers unions influence education outcomes (e.g. policymaking, student achievement) are constrained by the narrow way in which scholars conceptualize and measure union power. Specifically, I show that teachers unions primarily influence contemporary education policy-making and, by extension, student achievement through their clout in electoral politics, not through collective bargaining. Although bargaining laid the initial groundwork to more easily organize and mobilize teachers in politics during the 1960s and 1970s, over time, I find that the participatory returns provided by unionization diminished as teachers unions in non-mandatory bargaining states identified alternative ways to gain influence in the public policy-making process.