During critical realignments, citizens are able to reject past habitual behaviors to produce fundamental changes in the partisan balance. These realignments may be produced by any of three dynamics: the conversion of active partisans, the mobilization of inactive citizens, or the demobilization of active voters. Determining which dynamics have produced critical realignments is essential for understanding how citizens hold political elites accountable and forge nonincremental political change. This paper makes three central contributions to our understanding of these dynamics. In contrast to previous studies, we examine the dynamics of all presidential realignments in American electoral history. Where previous studies have focused on national, sectional, or state levels of analysis, we focus on city-and county-level realignments, a critical advancement for an inherently local-level phenomenon such as critical realignments. Finally, unlike previous studies, we identify the factors that promote particular realignment dynamics. We find that the conversion of active partisans has produced most of the enduring change in local electoral behavior in the United States, with the relative contribution of different dynamics varying both across time and space. Political factors such as the strength of state and local parties, social factors such as the strength of local social networks, and demographic factors, such as the sizes of local immigrant populations, have each favored particular realignment dynamics in American electoral history.