When and how did it happen?

[haele]

21. The failure of Reconstruction and rise of the Gilded Age.. Taft splitting from Roosevelt

Which sidetracked Roosevelt's initiatives for Environmental, Public Health, and Labor rights, Trust-Busting - and incremental Civil Rights.
Yes, Roosevelt was a bigot, but he was also a realist who had actual experience dealing with the causes of poverty and crime - and showed political willingness to incrementally introduce racial and gender equality measures even if he didn't personally agree with them.
This allowed Wilson to trash the "inconvenient" and expensive progressive policies to benefit Populist Capitalism and technological "progress" policies benefitting investors. Which led to Harding, and the rise of the Millionaire/Mob cabal dedicated to returning to the Gilded Age and Nativist policies to create a class system where only the owners would have "Democracy", and everything and everyone else was "owned".
They funded a lot of Anti-Communist and "Traditional" or Populist business groups to infiltrate and isolate small communities or community support organizations similar the KKK model without using that name - because the KKK had already lost widespread favor for their violent small-town isolation enforcement practices against white non-KKK members or visitors and their Anti-Catholic policies by the late 1920's. The rise. The goal was to polarize, isolate and capture a sufficient number of "in groups" while demonizing - and stealing community property and resources from - the out groups.

FDR, and to some degree presidents Truman through Johnson neutered them, but the Southern Strategy brought them roaring back