I think it is the other way around - the revulsion toward underclass and welfare dependency is the factor causing racism.
Negative attitude toward underclass and its behavior is nearly universal among human societies. The main reason for that is that most people tend to see immediately observable behavioral traits as indicators of personal characteristics - few venture into speculations about possible systemic causes of human behavior. In plain English, a person behaving politely is seen as "smart" and "worthy", whereas a persone behaviung rudely is viewd as "dumb" and "unworthy." Consequently, members of the underclass, who often tend to display socially undesirbale behavior, are seen as dumb and unworthy. As I already said, this is nearly universal - in almost every society on Earth the underclass behavior is scorned and despised.
Since in the US, the underclass coincides with ethnicity (black or hispanic), the aversion toward the underclass is the source of more general aversion toward members of the said ethnic group, a phenomenon known as racism. I can illustrate that with my observations of immigrants who changed their attitudes toward blacks. Before they came to this country, their image of blacks was that of jazz and blues musicians - and thus quite favorable. However, having lived in the poor sections of New York or Chicago, they have been exposed to black underclass. As a result, their attitude changed toward espousing racist stereotypes or even straightforward racism. Clearly, their personal experience of the underclass was a factor in that attitude change.
What is more, the negative attitude and stereotypes of Poles, Irish, or Italian generally decreased, as the members of these ethinc groups became assimilated to the mainstream society and stopped being tantamount with the underclass. Their role of being butts of jokes have been now taken by rednecks and trailer park inhabitants. That again demonstrates that attitudes toward an ethnic group changes with the change of that group's social-economic status.
wojtek