Excerpt from United States v. Lopez

First, Congress may regulate the use of the channels of interstate commerce. See, e.g., Darby, 312 U. S., at 114; Heart of Atlanta Motel, supra, at 256 (" `[T]he authority of Congress to keep the channels of interstate commerce free from immoral and injurious uses has been frequently sustained, and is no longer open to question.' " (quoting Caminetti v. United States, 242 U.S. 470, 491 (1917)).

Second, Congress is empowered to regulate and protect the instrumentalities of interstate commerce, or persons or things in interstate commerce, even though the threat may come only from intrastate activities. See, e.g., Shreveport Rate Cases, 234 U.S. 342 (1914); Southern R. Co. v. United States, 222 U.S. 20 (1911) (upholding amendments to Safety Appliance Act as applied to vehicles used in intrastate commerce); Perez, supra, at 150 ("[F]or example, the destruction of an aircraft (18 U.S.C. § 32), or . . . thefts from interstate shipments (18 U.S.C. § 659)").

Finally, Congress' commerce authority includes the power to regulate those activities having a substantial relation to interstate commerce, Jones & Laughlin Steel, 301 U. S., at 37, i.e., those activities that substantially affect interstate commerce. Wirtz, supra, at 196, n. 27.