“For the first time in American history, the federal government has attempted to ‘commandeer the people” by imposing on them an “economic mandate.” Such economic mandates cannot be justified by existing Supreme Court doctrines defining and limiting the powers of Congress. Upholding the power to impose economic mandates “would fundamentally alter the relationship of the federal government to the states and the people; nobody would ever again be able to claim plausibly that the Constitution limits federal power.”