The term lumpenproletariat was coined by Marx and Engels and denotes unproductive and regressive low-class parts of society - which are counter-revolutionary and often form the power base of reactionary politicians. I would argue that this term applies to large segments of the US society - not just the homeless or the junkies - regardless of race. If anything, I would say that the term probably does not apply to many homeless who are simply mentally ill and "left behind."
For more info see http://www.marxists.org/glossary/terms/l/u.htm also http://en.wikipedia.org/wiki/Lumpenproletariat
Wojtek