The Congress shall have Power

. . .

Clause 3:

To regulate Commerce with foreign Nations, and among the several States, and with the Indian Tribes;

. . .  And

Clause 18:

To make all Laws which shall be necessary and proper for carrying into Execution the foregoing Powers, and all other Powers vested by this Constitution in the Government of the United States, or in any Department or Officer thereof.
 

28 U.S.C. § 1652. - State laws as rules of decision

The laws of the several states, except where the Constitution or treaties of the United States or Acts of Congress otherwise require or provide, shall be regarded as rules of decision in civil actions in the courts of the United States, in cases where they apply.