Abstract: The constitutionality of the Affordable Care Act is sometimes said to be an "easy" question, with the Act's opponents relying more on fringe political ideology than mainstream legal arguments. This essay disagrees. While the mandate may win in the end, it won't be easy, and the arguments against it sound in law rather than politics. Written to accompany and respond to Erwin Chemerinsky's essay in the same symposium, this essay argues that each substantive defense of the mandate is subject to doubt. While Congress could have avoided the issue by using its taxing power, it chose not to do so. Congress has power to regulate commerce among the several States, but that might not extend to every individual decision involving economic considerations -- walking rather than taking the bus, stargazing rather than renting movies, or carrying a gun in a school zone rather than hiring private bodyguards. Even the necessary-and-proper power, the strongest ground for the mandate, may stop short of letting Congress claim extraordinary powers to fix the problems created by its exercise of ordinary ones. Because the mandate's opponents can find some support in existing doctrines, a decision striking down the mandate needn't be a drastic break from past practice. By contrast, a decision upholding the mandate would raise serious questions about the limits of Congress's powers. To many, these questions offer good reasons for doubting whether existing doctrine gets it right -- reasons having more to do with constitutional theory than political preference.