Nonsense; such a "Constitutional pledge" was not only nonexistent, but also quite impossible. There was no such thing as a legal property right in a slave within the jurisdiction of a free state, the Penumbral Emanation hocus-pocus of Dred Scott notwithstanding.
The slave states added hypocrisy to their other deadly sins when they demanded abrogation of the states' rights of the free states in this regard.
steve-b: Nonsense; such a "Constitutional pledge" was not only nonexistent, but also quite impossible.
From Article IV, Section II, Clause 3: "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."
Was not the ratification of the Constitution by the Northern states an agreement or pledge to uphold it including this part? Or maybe their fingers were crossed. This part of the Constitution was not cancelled until the 13th Amendment.
The violation of this part of the Constitution is cited in causes for secession by the Southern states. As CSA Vice President Alexander Stephens said in 1863:
He [Lincoln], by force of arms, attempts to compel a return to what he calls allegiance to the constitution. This, too, in the face of the fact of open and avowed violation of their constitutional pledges on the part of those northern States alluded to, all of which gave him a cordial support. The Union must be preserved. The South must be forced back to her constitutional duty at all costs and all hazards, but not a word has he ever uttered against those northern States in open rebellion against one of the most important provisions of the constitution, one at least without which it is well known the constitution could never have been adopted, and the Union would have never been formed. He has never intimated a desire that these States should ever return to their constitutional duty and allegiance, while all the power of the government is resorted to to compel us to do so.
I forgot to address this part of your argument.
The right to ignore a duty specifically enumerated in the Constitution does not fall under states rights. The states rights doctrine covers items not specifically enumerated or delegated in the Constitution. In areas covered by the Constitution, such as the return of fugitive slaves, the Constitution trumps state laws, not the other way around.