Washington’s response to the Covid-19 pandemic is upending one of the most durable patterns of American politics. Throughout history, national emergencies have led to a more powerful and centralized federal government and to the transfer of federal power from Congress to the executive branch. This time, the federal response rests largely on state and local government and private enterprise, with a wave of deregulation clearing the way. The Trump administration has seized no new powers, and Congress has stayed energetically in the game.

The...