Asked to describe his vision of democratic socialism, Bernie Sanders has often pointed to the New Deal and Franklin Roosevelt’s “economic bill of rights.” “Today, in the second decade of the twenty-first century,” Sanders declared in a June speech, “we must take up the unfinished business of the New Deal and carry it to completion.”

The invocation has stumped some people. Wasn’t Roosevelt one of America’s great liberals? Why is Sanders, a longtime independent, cribbing from a totem of the Democratic Party? Jake Altman’s new book, Socialism before Sanders: The 1930s Moment from Romance to Revisionism, gives us a better understanding of why Sanders — who was politicized as a college student in the 1960s by the Socialist Party — would latch onto FDR’s fabled program.

Catalyzed by mass strikes and worker unrest, the New Deal ushered in a rash of reforms that many in the Socialist Party (SP) felt shot a dose of socialism into American capitalism. No longer was the state simply the handmaiden of business, the breaker of strikes — now it would actually encourage unionizing. Changing with the times, many SP members recast themselves as the left wing of the New Deal coalition, often through trade union work. While Sanders, to his great credit, never joined the Democratic Party, his insistence that “economic rights are human rights” is a clear reverberation from the New Deal–era Socialist Party.