Between World War 2 and the rise of David Duke, how much cooperation was there between groups like the Ku Klux Klan on the one hand and groups like the American Nazi Party on the other?
More can definitely be said, but this answer might be of interest as it focuses on the shift to cooperation, and the opening portion does focus on the 'pre-Duke' period. In addition, this one looks at interactions between the groups prior to WWII, so outside your scope, but possibly also of interest.