["Health Services Research, Volume 61, Issue 3, June 2026. ", "\nABSTRACT\n\nObjective\nTo test whether national health insurance reforms under the Affordable Care Act (ACA) and state Medicaid expansions affected insurance coverage among American Indian and Alaska Native (AIAN) people and estimate comparative effects for other racial‐ethnic groups.\n\n\nStudy Setting and Design\nWe use a difference‐in‐differences approach that decomposes effects of national insurance reforms from state Medicaid expansions.\n\n\nData Sources and Analytic Sample\nData are from the 2011 to 2019 American Community Survey. The analytic sample includes respondents aged 19–64 who identify as non‐Hispanic AIAN, non‐Hispanic white (NHW), non‐Hispanic Black (NHB), or Hispanic and live in areas with a minimum of 50 respondents in each racial‐ethnic group for 2009–2013.\n\n\nPrincipal Findings\nMedicaid expansions were associated with a 9.0 pp. reduction in uninsurance (p < 0.001), and a 4.4 pp. increase in Medicaid coverage (p = 0.049) among AIAN respondents, with stronger effects among those who reported having Indian Health Services. However, national insurance reforms were not associated with AIAN respondents' insurance coverage. Similarly, Medicaid expansions rather than national reforms improved Hispanic respondents' insurance coverage. Among NHW and NHB respondents, both Medicaid expansions and national reforms improved insurance coverage.\n\n\nConclusion\nMedicaid expansions were the driving force behind the ACA's positive effects on insurance coverage among AIAN people between 2011 and 2019.\n\n"]