The powers not delegated to the United States by the Constitution, nor prohibited by it to the states, are reserved to the states respectively, or to the people.
-- Tenth Amendment
There is some belief that somehow the new health law (still called "bill" by many discussions) violates this provision. It is to be noted that by its terms, the Tenth Amendment specifically reminds not only the limits on the federal government, but that various powers are delegated to the U.S. and besides that prohibited to the states. See, e.g., Art. 1, sec. 10. But, the amendment is often used to focus on only half of the matter. As someone quoted Jefferson:
I consider the foundation of the Constitution as laid on this ground: That "all powers not delegated to the United States, by the Constitution, nor prohibited by it to the States, are reserved to the States or to the people" (10th Amendment). To take a single step beyond the boundaries thus specifically drawn around the powers of Congress, is to take possession of a boundless field of power, no longer susceptible to any definition.
Jefferson isn't exactly someone whose views on the matter should be taken as authoritative. He after all didn't much care for Marbury v. Madison or McCulloch v. Maryland, less sympathetic than Madison to federal power, which after all was a main impetus of the Constitution in the first place. Jefferson at various times didn't quite seem remember:
Wikipedia notes: "The Tenth Amendment is similar to an earlier provision of the Articles of Confederation: "Each state retains its sovereignty, freedom, and independence, and every power, jurisdiction, and right, which is not by this Confederation expressly delegated to the United States, in Congress assembled." After the Constitution was ratified, some wanted to add a similar amendment limiting the federal government to powers "expressly" delegated, which would have denied implied powers. However, the word "expressly" ultimately did not appear in the Tenth Amendment as ratified, and therefore the Tenth Amendment did not amend the Necessary and Proper Clause."
But, watch out when the Supreme Court says that:
The amendment states but a truism that all is retained which has not been surrendered. There is nothing in the history of its adoption to suggest that it was more than declaratory of the relationship between the national and state governments as it had been established by the Constitution before the amendment, or that its purpose was other than to allay fears that the new national government might seek to exercise powers not granted, and that the states might not be able to exercise fully their reserved powers.
Art. I, sec. 8 etc. does provide a list of powers, and this is an important matter since Congress has limited powers vis-a-vis states with much broader ones. The removal of "expressly" and the "necessary and proper" clause in effect gives more play to the joints. And, surely, the federal government has broad power, especially over national matters. But, the power is still fixed:
To make all laws which shall be necessary and proper for carrying into execution the foregoing powers, and all other powers vested by this Constitution in the government of the United States, or in any department or officer thereof.
Thus, even with this added gloss on enumerated powers, there is a limit to "foregoing powers" and those "vested in this Constituiton," not some general federal police power. The states have limits put on their power, but in the scheme of things, a lot is left, including a range of areas where we more often than not feel the brunt of governmental power. A core value of the 10A is that this federalist principle is reaffirmed.** As the Congress noted when introducing the BOR to the states:
THE Conventions of a number of the States, having at the time of their adopting the Constitution, expressed a desire, in order to prevent misconstruction or abuse of its powers, that further declaratory and restrictive clauses should be added: And as extending the ground of public confidence in the Government, will best ensure the beneficent ends of its institution.
This isn't a trivial matter. The First Amendment is itself "declaratory" in that Congress never really had the power to abridge the freedom of speech, except to the degree it did. That is, it references a fundamental right, one can say "natural" right, that was not removed by the enumerated powers in the Constitution except when there was some overriding state interest. Which remains, the word "no" in the First Amendment notwithstanding. All of the Bill of Rights in effect clarified the limits on government, some specific rights and possible abuse of powers deemed specifically worth dealing with explicitly. The Tenth Amendment in effect did this for the overall principle of federalism.
One of particular concern in recent years (with some excesses as shown in some immunity rulings) up to a point (whatever Thomas or some Cato Society view wants it to be*):
The regulation of an intrastate activity may be essential to a comprehensive regulation of interstate commerce even though the intrastate activity does not itself “substantially affect” interstate commerce. Moreover, as the passage from Lopez quoted above suggests, Congress may regulate even noneconomic local activity if that regulation is a necessary part of a more general regulation of interstate commerce. See Lopez, supra, at 561. The relevant question is simply whether the means chosen are “reasonably adapted” to the attainment of a legitimate end under the commerce power.
For good or ill, as applied in that case (concurrence by Scalia) to medicinal marijuana at least, this underlines (see the link) how the "individual mandate" and so forth works on federal commercial grounds alone. It is also a valid tax measure. The validity of the health law does not rob federalism of its power when appropriate. But, when appropriate, federal power is supreme. As here.
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* Thus, even Randy Barnett in the last link doesn't provide much hope, even though he thinks the Supreme Court precedent is wrong. Not that someone who brings up the specter of the federal government forcing gun ownership is totally worth our trust -- President Washington signed into a law that did just that, requiring members of the federal militia to own certain arms that I assume they all could not simply make at their home smith hut.
** [Update] The importance of the 10A is underlined by the fact that some Federalists (see, e.g., section on CJ Marshall in Mr. Justice) had a broad view of federal power. James Madison himself voiced the opinion that one reason why a BOR was necessary was that the necessary and proper provision potentially opens up a broad range of power that matched the range of purposes set forth in the Preamble.
Thus, the enumeration of certain powers in actuality had a somewhat limited value. The importance is underlined by something that once was deemed proper, but is now rejected even by those who support a broad view of federal power. That is, the idea that English common law crimes in effect were transferred over to the federal government as appropriate. So, there was no direct need to have a law against libel before charging someone for libel against the U.S.
There is still something of this when it comes to international matters, but much less so domestically. A national self-defense exception to concerns of governmental regulatory power (see, Mr. Justice, section on Justice Sutherland) suggests why your average John Yoo type support this sort of thing. Less so I. Potential in the document or not, once upon a time or now, a consistent limited government principle seems appropriate here.