In 1949 a book by Arthur Schlesinger Jr. first appeared that would launch a phrase that would live ever thereafter.

The title of his book was "The Vital Center," which ultimately became a catch phrase for a political phenomenon that was far from his intent, and in fact incurred the disapproval  of the Pulitzer Prize winning author of "The Age of Jackson" and "The Age of Roosevelt" series'.  

The title referred to the ideological struggle between democracy and totalitarianism.  Schlesinger let it be known that vital center in a domestic political context involved a muddle, a splitting of differences that was contrary to the case he was making for progressive politics.

Nonetheless the phrase and meaning stuck in a contrary context to the author's intent.  It involves the considered truism, substantiated repeatedly, that the average American voter resides in the center as opposed to the components of activist party politics.