And if Congress had passed legislation outlawing slavery are you saying that the Southern states would have meekly complied?
They could not have done such a thing without amending the constitution. In fact, "free states" were impossible without amending the constitution.
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 on Claim of the Party to whom such Service or Labour may be due.