On the afternoon of April 1, 1983, Barack Obama, then a senior at Columbia University, made his way into the Great Hall of Manhattan’s Cooper Union to attend a “Socialist Scholars Conference.” There Obama discovered his vocation as a community organizer, as well as a political program to guide him throughout his life. The conference itself was not a secret, but it held a secret, for it was there that a demoralized and frustrated socialist movement largely set aside strategies of nationalization and turned increasingly to local organizing as a way around the Reagan presidency — and its own spotty...