This deceptively simple argument has several constituent parts, as Roediger pulls from an updated classical Marxism, new labor history, new social history, and theories of racism to delineate the development of whiteness and connect it to the formation of the working class in the early 19th century. Critical to his argument is the expansion of Marxist theory to include race: he argues that race cannot be reduced to class, and that the two social categories work in dialectic with one another. From the new labor/ new social history, he pulls the insistence that working class people are historical agents, not dupes controlled by capital, and that they are thus complicit in the construction of their own racism. And he builds on existing theories of race, racism and slavery (most notably George Rawick) to show that racism developed as a response to early 19th-century capitalist development, of which slavery was a constituent part: nostalgia for a pre-industrial past, combined with frustration at their own proletarianization, resulted in white working-class racism and hatred toward black slaves as emblems of both preindustrial peasants and industrial capitalist workers. From there, the step to racism as a constituent element of white working class identity was a small one.
The rest of the book includes two case studies that Roediger largely pulls from secondary sources: blackface minstrelsy as a way to provide an outlet for white racial tensions and frustrations; and the story of how the black Irish adopted racism against blacks to assume a white American identity. In both cases, Roediger shows how racism both constructs and masks the blackness that is at the heart of whiteness. While I was surprised to find that such an innovative and hugely influential thesis is built on secondary evidence and conjecture more than empirical research, I think the case studies illustrate Roediger's argument well, and I really like his innovative use of developments in studies of race, slavery, and the working class that came out of the new social history.