Academic research on Christian nationalism has revealed a considerable amount about the scope of its relationships to public policy views in the US. However, work thus far has not addressed an essential question: why now? Research by the authors of this Element advances answers, showcasing how deeper engagement with 'the 3Ms' - measurement, mechanisms and mobilization - can help unpack how and why Christian nationalism has entered our politics as a partisan project. Indeed, it is difficult to understand the dynamics of Christian nationalism without reference to the parties, as it has been a worldview used to mobilize Republicans while simultaneously recruiting and demobilizing Democrats. The mechanisms of these efforts hinge on a deep desire for social dominance that is ordained by God - an order elites suggest is threatened by Democrats and 'the left.' These elite appeals can have sweeping consequences for opinion and action, including the public's support for democratic processes.