Consistent with this structure, we have identified three broad categories of activity that Congress may regulate under its commerce power. First, Congress may regulate the use of the channels of interstate commerce. 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. Finally, Congress’ commerce authority includes the power to regulate those activities having a substantial relation to interstate commerce, those activities that substantially affect interstate commerce.
Within this final category, admittedly, our case law has not been clear whether an activity must “affect” or “substantially affect” interstate commerce in order to be within Congress’ power to regulate it under the Commerce Clause. We conclude, consistent with the great weight of our case law, that the proper test requires an analysis of whether the regulated activity “substantially affects” interstate commerce.
We now turn to consider the power of Congress, in the light of this framework, to enact § 922(q). The first two categories of authority may be quickly disposed of: § 922(q) is not a regulation of the use of the channels of interstate commerce, nor is it an attempt to prohibit the interstate transportation of a commodity through the channels of commerce; nor can § 922(q) be justified as a regulation by which Congress has sought to protect an instrumentality of interstate commerce or a thing in interstate commerce. Thus, if § 922(q) is to be sustained, it must be under the third category as a regulation of an activity that substantially affects interstate commerce.