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“Not only was David Duke grooming him to sort of be his protege, but sort of much scarier and probably more powerful, Derek saw himself the same way.”

Derek is Derek Black, and if there were a royal family of white supremacy, he was its fresh-faced crown prince. His father is Don Black, the founder of the virulently racist website Stormfront. His godfather is David Duke, the former grand wizard of the Ku Klux Klan. And Derek’s inoffensive presentation of his white nationalism proved potent.

“One of Derek’s most lasting and damaging impacts on this white nationalist movement is that he convinced his father to scrub Stormfront of all racial slurs, all Nazi insignia,” said The Post’s Eli Saslow in the latest episode of “Cape Up.” “Derek thought the way [they were] going to reach more people is, instead of of using this kind of language, [they] need to play to this false, but unfortunately, very widely spread sense of white grievance that still exists in big parts of this country.”

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Saslow chronicles Black’s transformation from a golden boy of white nationalism to one of its most outspoken critics in his new book, “Rising Out of Hatred: The Awakening of a Former White Nationalist.”

“Still, in this country, about a third of white people believe that they experience more discrimination and more prejudice than people of color or Jews. And that is wildly, wildly off base by every statistical, historical, factual measure that we have,” Saslow told me. “But the fact that that much grievance exists in parts of white America means that these messages have real currency in parts of the country, and Derek was one of the people to figure that out.”

Once, white supremacy and its messengers were shunned. But then, as Saslow writes in his book, rhetoric employed by the Blacks and Duke started seeping into the political mainstream, reaching its nadir when Donald Trump secured the Republican presidential nomination in 2016. For Don Black, Saslow writes in his book, that moment was white nationalism’s coming out.

We have been trying to recruit these same disaffected whites that Trump is going after — it’s the exact same audience. Everybody may not to call themselves a white nationalist. That sounds a little scary, but it’s the same principle. He is tapping into the fact that race is still a huge part of identity. He’s accelerating the timeline of our movement by several decades by making many millions of people more racially aware. Before, nobody really knew what a white nationalist was. Now he’s given us this incredible platform.

Black’s conversion started after his enrollment at New College of Florida. There, the devoted white nationalist came face to face with real people from the groups he denigrated from afar. In the morning, Black would host his Stormfront radio show. And for the rest of the day, he was a regular college student, trying to blend in.

“He sort of made this really calculated choice to be a white supremacist activist on the radio every day and a regular college student in Sarasota,” explained Saslow. “He would call in to his daily radio show ... they would go on the air and rail against the minority takeover and, you know, say really hurtful things about IQ differential spreading the false science. And then he would walk back onto campus and he would sort of befriend whoever walked by. And part of that is that Derek had always been hugely curious about things and about people and also he had never in his life been interpersonally hateful. And that he didn’t believe that it was ever useful to have like a direct interaction with somebody where you were name calling or things like that, and he tried to make these ideas that he had that he’d been given and learned from very early on and in childhood he tried to make them about science and he tried to depersonalize them.”

But it all became too much. “This double life was taking a toll on him, especially because he had started to make real friendships ... particularly with one woman, Rose,” Saslow said. “Derek started to date her before realizing she was Jewish.”

Listen to the podcast to hear the rest of Black’s incredible story. How he tried to out himself as a white nationalist just as others were starting to find out the truth. How his friends tried different approaches to get him to see the truth. How his friend Allison matched his level of curiosity and used his reliance on empirical evidence to help guide him out of white nationalism. How the killing of Trayvon Martin pushed Black to break with everything. And how the mainstreaming of white nationalism pushed him to speak out.

“Once he saw that happening, he decided it wasn’t enough to speak up against this ideology once,” as he did in a letter to the Southern Poverty Law Center. “He decided that he needs to continue to [speak] up against it because at this moment in particular I think being silent is almost like being complicit, like the stakes are just too high.”