United States v. Wrightwood Dairy Co. is the earliest case I’ve come across to formulate the “substantially affects” test in terms of “activities.”
The power of Congress over interstate commerce is plenary and complete in itself, may be exercised to its utmost extent, and acknowledges no limitations other than are prescribed in the Constitution…Hence the reach of that power extends to those intrastate activities which in a substantial way interfere with or obstruct the exercise of the granted power. United States v. Wrightwood Dairy Co., 315 U.S. 110, 119 (1942).
The rationale of Wrightwood‘s extension of Congress’s commerce power supports the extension of that power, not just to volitional acts, but to anything that “in a substantial way interfere[s] with or obstruct[s] the exercise of the [commerce] power,” because anything doing that interferes with or obstructs a plenary power. If Congress cannot regulate a subset (omissions) of that which substantially affects interstate commerce then, contra Wrightwood, Congress’s Commerce Clause power isn’t plenary. Instead, it is limited to those regulations of interstate commerce that can be accomplished without regulating omissions.
To embrace the activity/inactivity distinction is thus to reduce Wrightwood to asserting, in the same paragraph, that the power of Congress over interstate commerce both is and is not plenary. This is not the mark of a distinction grounded in doctrine. Especially so since Wrightwood is the rare case that explains why (as opposed to merely reciting that) the commerce power extends to intrastate activities. It is thus essential to understanding what activities means. An explication of the term in tension with Wrightwood is therefore not persuasive.