In the main dissent from today's Supreme Court ruling upholding the Patient Protection and Affordable Care Act, four justices (Antonin Scalia, Anthony Kennedy, Clarence Thomas, and Samuel Alito) say the individual health insurance mandate goes beyond anything that has passed muster under the Commerce Clause before:
The striking case of Wickard v. Filburn, 317 U. S. 111 (1942), which held that the economic activity of growing wheat, even for one’s own consumption, affected commerce sufficiently that it could be regulated, always has been regarded as the ne plus ultra of expansive Commerce Clause jurisprudence. To go beyond that, and to say the failure to grow wheat (which is not an economic activity, or any activity at all) nonetheless affects commerce and therefore can be federally regulated, is to make mere breathing in and out the basis for federal prescription and to extend federal power to virtually all human activity.
Thomas goes further in a separate dissent, where he reiterates his longstanding position that the "substantial effects" test underlying Wickard "is inconsistent with the original understanding of Congress' powers and with this Court's early Commerce Clause cases." Quoting himself, he says the test "has encouraged the Federal Government to persist in its view that the Commerce Clause has virtually no limits."
Chief Justice John Roberts, who wrote the majority opinion, agrees with the dissenters that the mandate does not qualify as a regulation of interstate commerce (which is why his argument rests on the tax power instead):
The individual mandate...does not regulate existing commercial activity. It instead compels individuals to become active in commerce by purchasing a product, on the ground that their failure to do so affects interstate commerce. Construing the Commerce Clause to permit Congress to regulate individuals precisely because they are doing nothing would open a new and potentially vast domain to congressional authority. Every day individuals donot do an infinite number of things. In some cases they decide not to do something; in others they simply fail to do it. Allowing Congress to justify federal regulation by pointing to the effect of inaction on commerce would bring countless decisions an individual could potentially make within the scope of federal regulation, and—under the Government’s theory—empower Congress to make those decisions for him.
The other four members of the majority—Ruth Bader Ginsburg, Stephen Breyer, Sonia Sotomayor, and Elena Kagan—did not join this part of Roberts' opinion, since they have no such reservations about an all-encompassing Commerce Clause. But for what it's worth (not much, given how broadly the authority to regulate interstate commerce has been read so far and given how much can be accomplished under the tax power as it is understood by the Court), a majority of the justices continue to argue that the Commerce Clause is not a blank check.