I have spent some time trying to guess the genesis of American Leftism. I propose the German historical-critical Bible scholarship of the 19th Century. Devout Protestants — and America was a devout Protestant nation in the 19th century — had to deal with the fact that some of their long-held doctrines were clearly not true. (Moses did not write the five books of Moses in the Old Testament, for example. Mark was written before Matthew, and Q was written before either, for a New Testament example.)
This gave greater credibility to the intellectual children of the malicious Voltaire and the irreligious Jewish intellectuals mingling with goyim in a de-Ghettoed Europe. Sophmoric youth like Woodrow Wilson, and a few years behind him Norman Thomas, abandoned their allegiance to the tradition-bound Presbyterianism of their pastor fathers in favor of the ultimately empty promises of the Social Gospel. The teachings of the Popes of the time were on Socialism’s inconsistency with the inherent diginity of the human person, but the Catholics were persecuted in the United States through Blaine Amendments to such a degree that they set up their own Ghetto system complete with schools and hospitals. Catholic teaching was certainly not going to get widespread support among American intellectuals — Catholicism was thought to be the antithesis of intellectualism, mired as it was in rote ritual.
Fools like Wilson gave way to villains like Roosevelt, and the moral basis of limited government was replaced by a spirit of expediency and power. “Should it be done” was replaced by “could it be done.” And we see the final form in the current Alinsky Administration and the Brownshirt Congress of Nancy Pelosi and Harry Reid.