Slavery was abolished by the 13th amendment.
Don't you find it odd that the same people that had their hand in crafting the Declaration of Independence suddenly forgot what it meant when it came time to ratify the constitution? Slavery was indeed recognized by the constitution and legal.
You're mixing cause and effect. That amendment was the result of the principles of our Declaration. It did not suddenly appear out of thin air. First came the argument against slavery, as Abraham Lincoln articulated so well. Without the argument, the Thirteenth Amendment would not have come about.
Everything Abraham Lincoln did politically was based on the sentiments embodied in the Declaration of Independence.
Are you really trying to argue against those principles?