They call him the “race whisperer”, a black civil rights activist able to manipulate some of the most noxious far-right figureheads in the US. Now James Hart Stern has triumphed again – and it is possibly his most extraordinary accomplishment to date. Stern, 54, has emerged as the new leader of one of the largest and oldest neo-Nazi groups in the US – the National Socialist Movement.

Stern said he had gradually wooed the group’s longstanding leader before eventually seizing control. “As a black man, I took over a neo-Nazi group and outsmarted them,” he said.

Having assumed control of an organisation whose members wear SS-like uniforms that resemble those worn in Nazi Germany, Stern now intends to undermine it.

His opening move as NSM president has been to address a lawsuit against the neo-Nazi group by asking a Virginia judge to find it guilty of conspiring to commit violence at a notorious white nationalist rally in Charlottesville in 2017.

Facebook Twitter Pinterest Neo-Nazi leader Jeff Schoep (centre) Photograph: Jim West/Alamy Stock Photo

His next step will be to replace the far-right group’s website with Holocaust history lessons.

Not surprisingly, Stern’s intervention has invited comparisons to last year’s Spike Lee movie BlacKkKlansman, in which an African-American cop infiltrates the Ku Klux Klan during the early 1970s. (Lee’s movie was itself based on a true story.)

Stern, too, has form with the KKK, having taken down a major white supremacist organisation after befriending former “grand wizard” Edgar Ray Killen while they shared a prison cell.

The KKK leader, convicted in the Mississippi Burning killings of three civil rights workers, grew to trust Stern, who had been imprisoned for mail fraud, to the extent he gave the black activist power of attorney over his estate. In 2016 Stern was then able to use his legal discretion to dissolve the infamous Klan organisation that Killen once led.

It was Stern’s relationship with the KKK leader that caught the attention of the former president of the National Socialist Movement, Jeff Schoep. In 2014 Schoep approached Stern to discuss his relationship with the KKK leader, the first black man his organisation had reached out to since it had contacted the civil rights activist Malcolm X.

Facebook Twitter Pinterest Spike Lee’s film BlackKkKlansman told the story of Ron Stallworth, a black cop who infiltrated the KKK. Photograph: David Lee/AP

The two stayed in contact and Stern said that he frequently confronted Schoep over his views on Adolf Hitler, the Holocaust and white supremacism.

“From day one, I always told him: ‘I don’t agree with you; I don’t like you,’” Stern said. “I talked to him because I wanted to hope to change him,” he told the Washington Post.

Earlier this year Schoep came to Stern for legal advice on a lawsuit filed by a Charlottesville counter-protester against his group, following the 2017 Unite the Right rally in which a young woman was killed.

Stern described the man who had led the Detroit-based group for 24 years as being desperate for a way out, partly because of the repercussions of the Charlottesville lawsuit but also because he felt a lack of appreciation from his followers. When Stern offered to solve the problem by taking control of the Detroit-based organisation Schoep, he said, readily agreed.

Court documents dated 15 February confirm that Stern is both director and president of the NSM, although he insists that he does not plan to dissolve the corporation because he doesn’t want neo-Nazi followers to reinvent the group associated with Hitler acolytes.

Facebook Twitter Pinterest The National Socialist Movement’s former leader Jeff Schoep speaking during a white nationalist rally in Georgia last year. Photograph: Bita Honarvar/AFP/Getty Images

“Everything is out in the open. My plans and intentions are not to let this group prosper. It’s my goal to set some hard records right,” said Stern.

The unlikely takeover comes as traditional US neo-Nazi groups are being eclipsed by the more nuanced efforts of new alt-right leaders such as Richard Spencer or sidelined by the mainstream white-nationalist movement that has boomed in the wake of the 2016 presidential election. Reports had suggested that, before Stern’s power grab, tension was mounting within the organisation with a faction advocating a move towards a less violent, less explicit brand of neo-Nazism. Critics inside the party had complained it wanted to remain “a politically impotent white supremacist gang” and aired concerns over Schoep.

As Stern prepares for a fresh wave of controversy over his takeover, he said: “Say what you want about me. But I’ve done this twice now.”