The answer would require more space than we have here. The Socialist Party in the early twentieth century was a far bigger party in the United States than it is today, though for reasons too numerous to explain here, socialism never had as much influence in the U.S. working class as it did in Europe. We recommend readers start with Sharon Smith’s book Subterranean Fire: A History of Working-Class Radicalism in the United States (Haymarket Books, 2005).