Lincoln believed, and a great war was fought, over whether that promise of liberty, rightly understood, extended to all Americans. It did and it does. 

A few decades later, some very smart academics with some very advanced political science theories from Germany considered the American Constitution and found it lacking, archaic.

Hegelianism devalued the Founders as mere reflections of their time, historically conditioned by obsolete thought with little of relevance to modern conditions. The Constitution must be "living," unfixed and formless, were it to have relevance to industrial Man. 

Franklin Roosevelt said the Old Liberty was insufficient, calling for a "Second Bill of Rights" in his 1944 State of the Union. For Roosevelt and the Progressives, mere "political rights" were "inadequate to assure us equality in the pursuit of happiness." Government needed to be the provider of happiness to ensure it.