When the Supreme Court held in June 2012 that states were not required to expand their Medicaid program under the Affordable Care Act (ACA), it reset the politics of health reform at the state level, creating a natural experiment to understand implementation decisions. With a generous reimbursement from the federal government, economic benefits for businesses and health systems, and coverage for the previously uninsured, states had much to gain by expanding Medicaid. Yet, the political divisions which emerged during Congressional debate and passage of the ACA marked the implementation of the Medicaid expansion. With 18 states rejecting implementing the Medicaid expansion, what explains state decision-making and the Medicaid expansion? I find that partisanship matters, but the Medicaid expansion has had a destabilizing effect on the Republican Party; the institutional design, organized interests, waivers, and national events have influenced whether and how a state decides to implement the Medicaid expansion. Implementation decisions are complex social phenomena, influenced by several interacting factors. One common account of ACA implementation decisions has focused on the role of partisanship. While Democrats have embraced the Medicaid expansion, Republicans have a much more complicated connection with the policy. Of all the states that have rejected the expansion, all but one is has a Republican governor and majority control over both chambers of the legislature. Yet, more than forty percent of Republican governors and a third of Republican majority control state legislative chambers have voted to expand Medicaid. To examine the implementation of the Medicaid expansion, I conducted in-depth comparative case studies across three states. To facilitate case selection, I used fuzzy set/Qualitative Comparative Analysis (fsQCA), which identified three “pivot” cases, Arizona, Michigan, and Utah, for analysis. Each of the three Republican controlled states was on the brink of expanding or not their Medicaid programs. I collected two types of data to detect variations in implementation across the cases. First, I conducted semi-structured key informant interviews across each of the case study states. Second, I analyzed thousands of documents, including written and oral legislative testimony, legal material, and government and think-tank reports related to the expansion. Gubernatorial support was necessary, but not sufficient, to explain state decision-making for implementing the Medicaid expansion. Across each of these cases, the Medicaid expansion created divisions between traditional economic interests and ideologically driven factions within the Republican Party. Supportive Republican policymakers attempted to overcome these intra-party divisions by leveraging the idiosyncratic institutional design to pressure or bypass oppositional legislators. Additionally, organized interests influenced whether and how a state implemented the Medicaid expansion. The role of organized interests was moderated by the ability of these groups to stay unified, the timing of their mobilization in response to the Supreme Court decision, and the intensity of their policy preferences. Waivers gave supporters of the Medicaid expansion flexibility to pursue “reforms” to change the calculus of policymaker perception of Medicaid expansion and expand coalition formation within the legislature, while pushing the Medicaid program in more conservative directions. Lastly, the national political environment influenced whether and how a state implemented the Medicaid expansion. By 2017, each of the states had entered a period of stasis with Medicaid, only for the Trump administration and its shifting politics and policy around Medicaid to unsettle the programs, causing states to reevaluate and adapt their expansions to the new environment.