“The Constitution of the United States does not rule on the legality or illegality of Slavery.”
Lincoln must have thought otherwise, based on his first inaugural address:
“There is much controversy about the delivering up of fugitives from service or labor. The clause I now read is as plainly written in the Constitution as any other of its provisions:
No person held to service or labor 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 labor, but shall be delivered up on claim of the party to whom such service or labor may be due.
It is scarcely questioned that this provision was intended by those who made it for the reclaiming of what we call fugitive slaves; and the intention of the lawgiver is the law. All members of Congress swear their support to the whole Constitution — to this provision as much as to any other. To the proposition, then, that slaves whose cases come within the terms of this clause “shall be delivered up” their oaths are unanimous. Now, if they would make the effort in good temper, could they not with nearly equal unanimity frame and pass a law by means of which to keep good that unanimous oath?”
Note well: “the intention of the lawgiver is the law.”
again, if slavery was “enshrined in the Constitution”, how did more than half of the States of the Union outlaw the institution. If you read the Confederate Constitution, that is how you enshrine slavery in a Constitution. Again, our Constitution does not make slavery illegal, nor does it make it legal. It acknowledges the existence of slavery in the United States, and does make some provisions for it. The Constitution makes no pronouncement on the legality of secession. That, like the slave issue was left for later generations of Americans to decide. Those decisions have been made.