I credit much of their shift in opinion to the contemplation of the principles expressed in the Declaration of Independence. Most of them at the time knew it wasn't intended to apply to slaves, but then they started thinking "Why should it not be?"
If you look up the Freedom Cases filed in Massachusetts courts you realize their entire legal argument hinged on the Declaration of Independence. They *DID* apply the Declaration to the condition of Slaves.
BTW, the Dred Scott decision was loaded with errors of fact and law.
But not in regards to the essential point; That the Declaration was never intended to apply to slaves.
For example, it says blacks were nowhere citizens of the colonies or new states. In fact, there were at least five states, including NC!, where free blacks were legally full citizens entitled to vote. But Taney just ignored this fact because it didn’t fit his argument.
A Seemingly common tactic for many controversial Supreme Court decisions. Yes, many states had Free Blacks who were citizens and exercised the rights of citizens. Taney went too far in asserting that none were eligible. He should have stayed within the lines of what was known to be the law, but I think he was trying to make a political point with that overreach.
The decision was disaster on every level. Much like Roe, it was intended to use the power of the Court to impose a final solution of a political controversy. At which it comprehensively failed.
It failed to quell the opposition. They continued working for what they believed and their arguments eventually persuaded a majority. I will point out that the Congress felt the need to pass the 14th amendment to override it. Obviously that decision held some power till 1868.
ALL men are created equal, etc., etc. Pretty clear and not suspect to much interpretation.
If you accept it as the basic principle of America, and at the same time defend the practice of slavery, you have only two logical options.
1) You can deny that those who are enslaved are “really” men, heading off down the road that leads to Auschwitz.
2) You can decide that the Founders were mistaken. All men are NOT created equal. As for instance, Calhoun and Stephens defended so eloquently. Of course, once you accept this argument, there is no particularly logical reason to assume that black/white is the only divider between the unequals. Why not rich/poor, nobly vs. ignobly born, etc.
Which of course gets us right back to the world most of humanity has always lived in.
I personally think that the DoI had a short-term negative effect on the interaction of Americans with both blacks and Indians. Since both were obviously not equal to white men, they couldn’t really be men at all, so must be some species of animals with no rights that “real” men had any obligation to respect, as Taney put it.
Only two categories allowed, men and not-men. And not-men could never become men, as Taney also said.
Meanwhile, the Spanish tradition provided for an infinite variety of gradations of status and freedom and rights between the slave and the King. In Latin America the slave or native could be viewed as simply an inferior man, not as some sort of talking animal.