According to a different source, this question refers to the late 19th century and early 20th century.
During this period, debates over immigration and assimilation in the United States continued, and intensified. This was mainly due to the fact that the country received an enormous influx of immigrants. The high levels of immigration led to deep changes in society, and to a new definition of what it meant to be "American." America became associated with the idea of a "melting pot."
During this period, class identities changed greatly as well. The country became more urban, as many people moved to cities to work in the new businesses and factories that were growing all over the country. Moreover, most immigrants settled in cities as well. The growth of cities gave rise to the working class, and the urban poor, altering the way class identities operated in the country.