The answer to the question of what do we do about it is difficult. However the issue of how we got here is relevant in framing the what we do about it, once we develop some stratigic vision of a political mechanism for the fix.
Like Korth, I started with a pro-Lincoln Republican party view--at this point, I agree completely with those who date the downfall of the republic from Lincoln's presidency.
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.