The clause gives Congress the power to regulate interstate commerce and, by implication, denies the states the power to regulate or impede interstate commerce. States may, then, only control intrastate commerce. As well, the Commerce Clause grants only the federal government the power to regulate trade with foreign nations, including effecting treaties, enacting embargoes, and conducting foreign relations. State regulation of imports or exports with foreign nations is invalid if it restricts trade or requires any special treatment of foreign goods. The states may, however, exercise limited regulation over foreign goods once those goods are in the hands of a retailer and the bulk of a wholesale lot has been broken down into smaller commercial units.
State regulation is invalid if it discriminates between domestic and foreign commerce.
Legislation covering such additional matters is valid where it makes regulation of an enumerated power more effective. This limitation prohibits the states, but not the federal government, from passing statutes or administrative regulations that modify the duties owed under existing contracts. The clause was included primarily because at the time of enacting the Constitution, more citizens were debtors than creditors and the framers were concerned that the debtors might elect legislators based upon a promise to pass laws which would modify debt contracts. Without this recognition, some of the states could become a refuge for persons seeking to evade their legal obligations elsewhere. The freedom of speech provides society with open access to the general marketplace of ideas.