After President Barack Obama signed into law the Affordable Care Act (ACA) of 2010, 26 states filed lawsuits challenging key provisions. In 2012, the Supreme Court ruled in support of most provisions but held that the mandatory Medicaid expansion was unconstitutional, in effect making it optional for the states. Yet Medicaid expansion is critical if the ACA is to achieve its goal of substantially increasing coverage, especially for lower-income adults with health problems, many of whom are racial and ethnic minorities. This article examines the factors explaining state variation in Medicaid expansion from 2012 to 2014. Although partisan power in the states is a key predictor, other relevant factors include the state's Medicaid policy legacy, providers' influence, conservative ideology, and racial resentment. We demonstrate that the enactment of legislation is only a part of the policy formation process and that implementation is equally salient, with state dynamics playing a critical role.