The passage of the Patient Protection and Affordable Care Act (ACA) in 2010 represented the most significant expansion of access to healthcare and health insurance in the modern United States. Support for the ACA, as well as a major component Medicaid expansion, was significantly dictated by partisan affiliation; Republicans largely opposed the bill while Democrats largely supported it. Nevertheless, a significant number of Republican-led states took up ACA expansion throughout the 2010s; in 2017, Republican efforts to repeal the ACA were ultimately unsuccessful with the legislation failing in the Republican-controlled U.S. Senate. Concurrent with these political debates, the most significant drug epidemic in the history of the United States was under way in the form of the opioid crisis. At its onset white non-Hispanic individuals (WNH), frequently of lower- or middle- socioeconomic class, were most heavily impacted by substance use disorder and opioid use disorder (SUD/OUD). This demographic also overlapped significantly with the Republican party’s demographic base. Using process tracing and semi-structured interview methods, this research seeks to understand decision-making by both Republican governors and Democratic governors in conservative states (determined utilising Cook Partisan Voting Index data) concerning implementation of ACA expansion throughout the 2010s. It also seeks to understand how political incentives influenced voting by the Republican senators who dissented from their party leadership during the 2017 ACA repeal vote. Finally, it will explore perceptions of whether Medicaid expansion helped create or fuel the opioid epidemic, as well as impressions of the ACA’s relevance to current SUD/OUD treatment systems. The dissertation finds that the opioid epidemic likely had little effect on Republican state decision-making concerning ACA expansion through the 2010s, with the exception of Ohio where it was a significant driver of ACA adoption. The epidemic had a more significant impact on political decision-making in the 2017 ACA repeal effort, with several Republican senators electing to vote against repeal partly due to impacts the elimination of Medicaid expansion would have on their states’ ability to combat the opioid epidemic. No thought leaders at the state or national level believed Medicaid expansion fuelled the opioid epidemic in any meaningful way. All interviewees considered the ACA critical to current SUD/OUD treatment systems at the state and federal level.