I would say that before Lincoln America was a fairly loose confederation of soverign states. He helped make us one country with a common currency (introduced in 1861) and, far more important, overarching national loyalties.
The constitution is not a religious document. Madison referred to bills of rights, in general as being "parchment promises" -- easily made, easily broken. If Lincoln had followed constitutional writ without exception, Maryland would have been ungovernable and Washington, D.C. lost. No President, not then, not now, will ever allow that merely because of a generalized pledge to uphold our often vague constitution. Practical people can find a way to uphold principal as much as possible without allowing the constitution to be a suicide pack, and that is what Lincoln did. Conservatives should set high standards, but not utopian standards no real flesh and blood politician will follow.
There is no point in having a written Constitution if it is not going to be obeyed by government officials. If "flesh and blood" politicians take the oath to the Constitution, they are bound to follow the Constitution, whether they like its provisions or not. And if "flesh and blood" politicians find parts of the Constitution objectionable, they can work to amend by the processes provided by the Constitution itself.