Congress originally tried to defend the constitutionality of ObamaCare’s individual mandate (which would require all Americans to buy health insurance) based on its power to regulate interstate commerce, but it has since gotten cold feet and has switched to a defense based on its power to tax. But not buying health care isn’t an aspect of commerce, and a penalty for not buying health care isn’t a tax — and for the Supreme Court to uphold the mandate on either basis would require the Court to allow Congress to go where it has never gone before.