Commerce Clause
Congress's power to regulate interstate commerce — the source of most federal regulatory authority.
On the MBE, the affirmative-power “tell” is a federal statute reaching some local activity: look first for aggregation — Congress may regulate purely intrastate, even non-commercial conduct if, taken in the aggregate, it substantially affects interstate commerce (Wickard v. Filburn, homegrown wheat; Gonzales v. Raich, homegrown marijuana). The trap is the answer insisting the activity itself cross state lines — it need not. But the NFIB v. Sebelius line limits this: Congress may regulate existing economic activity, not compel inactivity (it cannot force you to buy insurance), and the substantial-effects category cannot reach noneconomic activity by stacking attenuated effects.
For dormant Commerce Clause patterns, the hinge is discriminatory versus merely incidental burden: facial discrimination is virtually per se invalid (survives only if narrowly tailored to a legitimate local end with no nondiscriminatory alternative), while an evenhanded law gets Pike balancing (burden weighed against local benefits). Two escape hatches students forget: the market-participant exception (a state buying or selling may favor its own) and congressional consent, which cures an otherwise-invalid burden. Don’t confuse this state-restraining doctrine with anti-commandeering or Eleventh Amendment immunity from the sovereignty material.
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