The US Constitution is the Bean Counter for the Declaration. It is about the nuts and bolts of running a nation, and nothing else. It's authority is derived from the Legitimate authority given it by the Declaration of Independence, and it has no moral authority of it's own.
In simple terms, the foundational principles outlined in the Declaration of Independence trump the "rule book" which is the US Constitution.
Which once again I must comment, your justification for secession directly contracts the justifications offered by the men who actually seceded 150 years ago. They all rejected the Declaration while Lincoln made it the center piece of his philosophy.