The Constitution devotes the national domain to union, to justice, to defence, to welfare and to liberty. But there is a higher law than the Constitution.
Southerners simply wanted the Constitution obeyed, which the Northern states were not doing. Seward, having reaped the whirlwind with his earlier 'higher law' statement, finally backtracked and said the following in early 1861:
I agree that all laws of the States, whether free States or slave States, which relate to this class of persons, or any others recently coming from or resident in other States, and which laws contravene the Constitution of the United States, or any law of Congress passed in conformity thereto, ought to be repealed.