A clause in the United States Constitution that empowers Congress to enact all laws “necessary and proper” to carry out the powers of the federal government under the Constitution. See U.S. Const. art. I, § 8, cl. 18 (“The 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 clause is not limited to laws that are essential or indispensable; rather, it gives Congress discretion to adopt any convenient or appropriate means for accomplishing any constitutionally permissible objective.
United States v. Kebodeaux, 570 U.S. 387 (2013):
The scope of the Necessary and Proper Clause is broad. In words that have come to define that scope Chief Justice Marshall long ago wrote:
“Let the end be legitimate, let it be within the scope of the constitution, and all means which are appropriate, which are plainly adapted to that end, which are not prohibited, but consist with the letter and spirit of the constitution, are constitutional.” McCulloch v. Maryland, 4 Wheat. 316, 421, 4 L.Ed. 579 (1819).
As we have come to understand these words and the provision they explain, they “leav[e] to Congress a large discretion as to the means that may be employed in executing a given power.” The Clause allows Congress to “adopt any means, appearing to it most eligible and appropriate, which are adapted to the end to be accomplished and consistent with the letter and spirit of the Constitution.”
The Constitution, for example, makes few explicit references to federal criminal law, but the Necessary and Proper Clause nonetheless authorizes Congress, in the implementation of other explicit powers, to create federal crimes, to confine offenders to prison, to hire guards and other prison personnel, to provide prisoners with medical care and educational training, to ensure the safety of those who may come into contact with prisoners, to ensure the public’s safety through systems of parole and supervised release, and, where a federal prisoner’s mental condition so requires, to confine that prisoner civilly after the expiration of his or her term of imprisonment.
United States v. Mujahid, 799 F.3d 1228 (9th Cir. 2015):
“[T]he Necessary and Proper Clause grants Congress broad authority to enact federal legislation.” United States v. Comstock, 560 U.S. 126, 133, 130 S.Ct. 1949, 176 L.Ed.2d 878 (2010). In general, a statute is within the scope of Congress’ authority as long as it “constitutes a means that is rationally related to the implementation of a constitutionally enumerated power.” Thus, although “ ‘Congress cannot punish felonies generally,’” it is well established that Congress may create federal crimes under the Necessary and Proper Clause . . . In Comstock, applying these principles, the Supreme Court upheld a federal civil-commitment statute authorizing the continued detention of “a mentally ill, sexually dangerous federal prisoner beyond the date the prisoner would otherwise be released.” The Court concluded the statute was “a ‘necessary and proper’ means of exercising the federal authority that permits Congress to create federal criminal laws, to punish their violation, to imprison violators, to provide appropriately for those imprisoned, and to maintain the security of those who are not imprisoned but who may be affected by the federal imprisonment of others.” In reaching this conclusion, the Court considered five factors: “(1) the breadth of the Necessary and Proper Clause, (2) the long history of federal involvement in this arena, (3) the sound reasons for the statute’s enactment in light of the Government’s custodial interest …, (4) the statute’s accommodation of state interests, and (5) the statute’s narrow scope.” Id.