On the eve of the War of 1861, even unionist politicians saw secession as a right of states. Maryland Rep. Jacob M. Kunkel said, "Any attempt to preserve the Union between the States of this Confederacy by force would be impractical, and destructive of republican liberty." The northern Democratic and Republican parties favored allowing the South to secede in peace. I'm not sure you can use Maryland politicians as a guide to the beliefs of northern political leaders. The first suspension of habeas corpus by Lincoln was for the imprisonment of Maryland secessionists.
And the second was a Supreme Court Justice. Lincoln was a tryannt.