Alongside growing controversy over judicial nominations, court reform and Covid-19 policies, American law is in the midst of a little-noticed paradigm shift in courts’ treatment of public health measures.

The Republican Party’s campaign to take over the federal and state courts is quietly upending a long and deeply embedded tradition of upholding vital public health regulations. The result has been a radically novel and potentially catastrophic sequence of decisions blocking state responses to the coronavirus pandemic.

For centuries, American constitutional law granted state governments broad public health powers. “Salus populi suprema lex,” the old saying went: The health of the people is the supreme law. Such authority went back to the beginning of the Republic. In the famous 1824 case of Gibbons v. Ogden, Chief Justice John Marshall defended the “acknowledged power of a State to provide for the health of its citizens.” States, he explained, were empowered to enact “inspection laws, quarantine laws” and “health laws of every description.”

Lemuel Shaw of Massachusetts, who was arguably the most respected […]