Actually, I'd say it was the KKK in the 1920s, at least in America. Where the KKK failed was in that it was, fundamentally, a secret society, while fascism is a mass movement, and a secret society so narrowly focused (no Catholics, no blacks, no immigrants, no unions) that by the time it reached its greatest power, it had, in essence, ideologued itself right out of competition. It never made the transition to political movement precisely because it lacked the ability to reach out beyond its white, Anglo-Saxon, Protestant base, which at the time was steadily shrinking. The American "nation" is not, and never has been, primarily defined by its ethnic or religious identity, and that is where the Klan failed.
I'd actually say that the American nation defined itself for a long time as a white Anglo-Saxon Protestant one - especially in the South.
I can imagine a timeline where more imported slaves led to more majority black states in the South. This causes more exposure to brutality faced by blacks and a stronger Radical Republican bloc. After the Civil War, these Radical Republicans strike a tougher line on reconstruction, and you get African-Americans running state governments. Just have one of these elect a leader similar to the African anticolonial leaders of OTL, and you get a huge white backlash nationwide. Populist leader emerges hating foreign "African" elements, the individual rights theories of the Republicans, and the wealthy Northern bankers that fund them. Takes power by legal or illegal means, dismantles system of government and writes a new one.