Because I believe that the way this concept is used in political discourse, it is based on "blue-collar" cultural identity and populist vernacular, rather than on the relations to the means of production. Otherwise, the distinction between 'working class" and 'yuppies' - which is quite common on the left - would make no sense whatsoever.
To define a concept it is important to enumerate its connotations, its designates, as well as its contrasting notions. The concept 'working class' - at least as it is used in the US - connotes an unmistakable anti-intellectualism that some argue (cf. Hofstadter) permeates the US culture. It is contrasted with the 'cultured' and the 'educated' which in popular discourse is often associated with elites, urbanism, and mental work. Therefore, a Latino janitor - or for that matter Black drug dealer - is a member of the working class, but an Indian or Russian software engineer is not by the virtue of their education and an connection to culture (work with their heads rather than their hands).
Stated differently, 'working class' - as used in this country - is an emotive rather than an empirical concept - it expresses certain emotions, attitudes and cultural values (such as anti-intellectualism and populism) rather than connotes an empirically identifiable feature that identifies a class of individuals regardless of their cultural or ideological traits.
PS. I heard a story, perhaps an urban legend, that during the Bolshevik Revolution, armed gangs were challenging people on the streets asking them to "show their hands" ("pokhazhi ruki"). If their hands did not have signs of manual labor, they were beaten or shot. I also know for the fact, that similar attitudes toward mental labor were expressed by the Cultural Revolutionaries in China - I personally witnessed "trials" of teachers and college professors by mobs of Red Guards.
Wojtek