Mr. Regnery attended the University of Pennsylvania, where he studied political science and joined the Intercollegiate Studies Institute, a conservative student organization co-founded by Mr. Buckley. He left before graduating to work on Senator Barry Goldwater’s 1964 presidential campaign.

In the 2017 interview with Buzzfeed, one of the few times he spoke to the news media, he claimed that his efforts on behalf of Mr. Goldwater included what he called “Operation Dewdrop,” in which he attempted to deter Democratic voters in Philadelphia by hiring a plane to seed the skies with dry ice, in the hopes of making it rain. He failed — though, he recalled, he burned his fingers on the ultracold dry ice containers.

Mr. Regnery later returned to Chicago, where he worked for Joanna-Western Mills. He became the company’s president in 1980 but was ousted a year later, after several quarters of poor financial performance. According to his own account, he spent the rest of his career in a variety of businesses, while also dabbling in Illinois politics.

In his memoir, he recounted how he first began to turn against the Republican Party after listening to a speech in 1993 in which the economist Milton Friedman declared that the end of the Cold War meant that the free-market economic doctrines of the Reagan era had won. In an early sign of that break, according to a 2017 profile in Mother Jones, Mr. Regnery ran unsuccessfully for Illinois secretary of state in 1994 on the Term Limits and Tax Limits Party ticket

Five years later, he convened a Who’s Who of white supremacists for a conference in Florida, where he delivered a speech, “For Our Children’s Children,” in which he said the only way to save America’s white identity was for it to break up into several smaller countries, one each for the country’s various ethnic groups.

His racism grew more explicit. He announced plans in 2004 to start a whites-only dating site. It never happened, but he continued to worry that white people were in danger of extinction: In 2006 he delivered a speech in Chicago in which he said, “The white race may go from master of the universe to an anthropological curiosity.”

By then he had severed most of his ties with mainstream Republicans, and they with him. T hat same year the leadership of the Intercollegiate Studies Institute, which he had joined in college, removed him from its board.