The ACA (2010) originally required all states to expand Medicaid; the Supreme Court in NFIB v. Sebelius (2012) made expansion optional. As of 2024, 41 states and DC have adopted expansion; 10 states have not.
States that expanded saw substantial reductions in uninsured rates, improvements in health outcomes for low-income adults, and reduced uncompensated-care costs at hospitals. Non-expansion states have a "Medicaid gap": adults too poor for ACA marketplace subsidies but ineligible for Medicaid under their state's pre-ACA rules.
Defenders argue expansion is one of the largest poverty-reducing health-policy interventions in decades. Critics argue Medicaid coverage doesn't deliver good health outcomes per dollar and creates state budget risks if federal match shrinks.