“As enacted, the ACA Medicaid expansion was a mandatory expansion of Medicaid eligibility to non-elderly adults with incomes up to 133% of the federal poverty level (FPL). However, on June 28, 2012, in National Federation of Independent Business v. Sebelius, the U.S. Supreme Court found that the federal government could not withhold payment for a state’s entire Medicaid program for failure to implement the ACA Medicaid expansion. Instead, the federal government could withhold only funding for the ACA Medicaid expansion if a state did not implement the expansion, which effectively made the expansion optional.
“After the Supreme Court ruling, the Centers for Medicare & Medicaid Services (CMS) issued guidance specifying that states have no deadline for deciding when to implement the ACA Medicaid expansion. The guidance also stated that states opting to implement the ACA Medicaid expansion may end the expansion at any time.”
Source: U.S. Congressional Research Service. In Focus: Overview of the ACA Medicaid Expansion (IF10399; Dec. 3, 2018). https://crsreports.congress.gov/product/pdf/IF/IF10399