Commerce Clause\Necessary and proper\Taxing Spending- Funding clauses.
for instance: Article 1 Section 8 clause 3
Congress has power to regulate commerce with foreign nations, and among several states, as well as with Indian Tribes
states can't regulate commerce. (course, try and take fruit to and from california\arizona)
Course, Congress has abdicated so much and the Judiciary has made so many laws, being a strict constitionalist has no meaning.
Sad, but true.
As a poster mentioned on another thread his morning, "we live in a post constitutional country"