Charles basic point is surely correct, and certainly the slavemasters of each Sovereign State relied upon a slavemaster unity that transcended the course of the Savannah River (or any other State boundary) in the event of any locally successful revolt. But technically speaking Article 4 Section 2 of the Holy and Sacred Slavemasters Constitution of these United States of America ("No Person held to Service or Labour in one State, under the Laws thereof, escaping into another, shall, in consequence of any Law or Regulation therein, be discharged from such Service or Labour, but shall be delivered up ...") gave a Constitutional basis totally independent of the Supremacy Clause for Fugitive Slave Laws.
In fact they weren't able to enforce the Fugitive Slave Law in many non-slave states even with their Blessed and Hallowed Constitution, Supreme Court, federalism and Article 4 Section 2... And it was (for instance) the courts of Wisconsin that justified their refusal to enforce the Fugitive Slave Law by reference to "States Rights."
john mage