There's a big difference between moving toward the Declaration as an ideal as we still were in the time from the Founding until the early 1900s vs completely running away from those ideals as we have since.
This also holds true for all American political history from the founding of the country to the Civil War.
The Civil War started right after the industrial revolution ended. By this time, the balance of power was greatly in favor of the North.
The amazing thing about the Founders, though, was that the admirable forces were able to write the documents.
Compare that to today's "gang of eight" senators who created the immigration bill. That thing was a disaster before they had their first meeting. They had a unique chance to create solid immigration law, but have f'd it up beyond all reasoning.
The Founding documents were philosophical and political works of art. Where are works such as they coming form in today's society? Where are they being enacted into law?
Even the founders debated whether to abolish slavery, but ultimately punted because they felt uniting the states was more important.
That is, it was a tough compromise.
Take a look at the book The Long March (about Lincoln). There was a several-decade long movement before 1864 to abolish slavery.
The book also explores the tensions between slavery and American ideals, and the many complications the tensions produced.
Doesn't slavery demonstrate that this ideal never even existed in the first place?