Under the United States Constitution, Congress has limited powers:  Specifically, it only can do those things that are specifically enumerated in Section 8.  However, the last paragraph of Section 8 (¶18) states: 

[Congress shall have power] 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. 

This is the so-called “Necessary and Proper Clause.” 

In Federalist No. 44, James Madison defended the Necessary and Proper Clause:

Without the substance of this power, the whole Constitution would be a dead letter. …  No axiom is more clearly established in law, or in reason, than that wherever the end is required, the means are authorized; wherever a general power to do a thing is given, every particular power  necessary for doing it is included.” ***

If Congress exercises powers not warranted,  ”In the first instance, the success of the usurpation will depend on the executive and judiciary departments, which are to expound and give effect to the legislative acts; and in the last resort a remedy must be obtained from the people who can, by the election of more faithful representatives, annul the acts of the usurpers.”  James Madison, Federalist No. 44.

And according to Professor Randall E. Barnett:

To determine the constitutionality of any particular legislation and evaluate judicial applications of the Commerce Clause, however, we must also consider the meaning of the Necessary and Proper Clause. If the original meaning of “proper” in this clause was, as Gary Lawson and Patricia Granger have shown, that the end or purpose of a law must be within the jurisdiction of Congress to enact, a narrow conception of the Commerce Clause limits Congress to the end or purpose of making regular the trade between the states. Legislation that is actually for a different purpose cannot be upheld as “proper.” As Chief Justice John Marshall stated in McCulloch v Maryland, “should Congress, under the pretext of executing its powers, pass laws for the accomplishment of objects not entrusted to the government, it would become the painful duty of this tribunal . . . to say that such an act was not the law of the land.”

And, as I have argued elsewhere, if the Necessary and Proper Clause requires an assessment of “necessity” in which legislation is scrutinized to determine if there is adequate fit between means and ends, then the Congress must show, at minimum, it has chosen means that actually conduce to an enumerated end. Even John Marshall, who construed the degree of necessity required by the clause more loosely than I think is warranted, allowed that the means chosen must be “plainly adapted” to a “legitimate” end that is “within the scope of the constitution.”  In which case, the only “appropriate means” are those that are actually incidental to making regular trade between the states.

T. Jeffrey Beausay