Why did Chief Justice Roberts describe the threatened loss of Medicaid funding as a “gun to the head”?

In the Affordable Care Act cases, Chief Justice John Roberts used the phrase “gun to the head” to describe the coercive nature of the federal government’s threat to withhold all Medicaid funding from states that chose not to participate in the program's expansion.1

Under the original provisions of the Patient Protection and Affordable Care Act, the Secretary of Health and Human Services was authorized to strip states of all federal Medicaid funds—including those supporting existing beneficiaries—if they refused to expand their Medicaid rolls to cover new populations. Roberts argued that because states rely on these enormous sums of money to provide essential services, the threat of losing them was so severe that it effectively removed any voluntary choice, amounting to compulsion rather than a legitimate exercise of federal spending power.12

To remedy this, the Supreme Court ruled that while the expansion itself could stand, the federal government could not withhold funds for existing Medicaid beneficiaries from states that declined to participate in the expansion.13