Leonard Zeskind, who tracked right-wing hate groups and who foresaw that anti-immigrant ideologies would move to the mainstream of American politics, died on April 15 at his home in Kansas City, at 75.

Mr. Zeskind spent decades studying white nationalism, documenting how its leading voices had shifted their vitriol from Black Americans to nonwhite immigrants. His 2009 book, Blood and Politics: The History of the White Nationalist Movement From the Margins to the Mainstream, resulted from years of following contemporary Klansmen, neo-Nazis, militia members, and other right-wing groups. His investigations earned him a MacArthur “genius grant” in 1998.

At a 2018 town hall meeting in Washington, Mr. Zeskind called on Democrats in Congress to oppose a bill sponsored by Rep. Steve King of Iowa to end birthright citizenship — that anyone born in the United States is a citizen — a focus of anti-immigrant groups warning of threats to the “white race.”           Recently, his book was one of 381 removed from the U.S. Naval Academy library in a purge of titles about racism and diversity, The New York Times reported.