The KKK had no connection with leftism. The other groups did. And Eric Foner, who is cited as an authority by Jeffery Lord, is a Marxist:
https://historyhalf.com/eric-foner-preaches-marxism-on-campus/
BTW, you still a neverTrumper? Or did you give that up?
Oh, but the KKK did indeed have a connection to the progressives. For one thing, there is Margaret Sanger, who spoke to their woman’s auxilliary.
https://www.liveaction.org/news/planned-parenthood-founder-spoke-to-kkk-but-photo-is-fake/
Yes. Foner is indeed a Marxist.
It's similar to his citing of the left-wing PBS for the proposition that "Klan evolved into a terrorist organization. It would be responsible for thousands of deaths, and would help to weaken the political power of Southern blacks and Republicans."