The state of Arizona became the main corridor for unauthorised migration into the United States in the early 2000s. A security approach to the issue at the state and local levels of policymaking became increasingly marked later in that decade. This escalation challenged the longstanding settlement in the United States that immigration was an exclusively federal matter, but occurred during a time when, by traditional measures, the unauthorised entry problem was easing. Such a development raises important questions about why security is chosen as a policy approach, highlighting the need to understand the securitization of immigration as a matter of political rationality. This thesis uses recent immigration politics in Arizona as a case study in order to examine why policymakers treat an issue like immigration as a security issue, when other interpretations are available. This thesis provides a detailed historical narrative of the evolution of migration and border-security politics at these levels of government in Arizona from 2004, when a broad political consensus began to emerge that there was a security problem on the international border which the state had to act to address, to 2011, when the then-years-long trend of securitizing immigration at the state level was abruptly halted. Taking an interpretivist approach to understanding policymaking, this thesis employs semi-structured elite interviews with state and local-level policymakers in Arizona, and extensive analysis of media and government documents. This research contributes originally to knowledge in two main ways. First, it furthers the migration politics field by advancing its understanding of the securitization of migration, and particularly the phenomenon of parties across the political spectrum coming to support security approaches towards, and restriction of, immigration. This thesis thoroughly explains the occurrence of this phenomenon in a major case, identifying the elite political logics, strategies, and understandings that were instrumental in the decisions that composed this process. Second, this thesis contributes to a developing security-studies literature that conceptualises securitization not as an “exceptional” form of politics, but as driven by “normal” political considerations. This research identifies how competitive democratic political logics produced phenomena usually ascribed by securitization theory to exceptionalism, including the narrowed field of contestation around security issues, and the adoption of policies that would previously have been considered extreme. It also examines how, in this case, securitization was successfully contested democratically. In this way, this thesis contributes toward the development of a concept of “security politics.”