Cato's Daniel Griswold argues that the influx of immigrants actually increases the income and skill set of native-born Americans:
One argument raised against expanded legal immigration has been that allowing more low-skilled foreign-born workers to enter the United States will swell the ranks of the underclass. [...] As plausible as the argument sounds, it is not supported by the social and economic trends of the past 15 years. Even though the number of legal and illegal immigrants in the United States has risen strongly since the early 1990s, the size of the economic underclass has not. In fact, by several measures the number of Americans living on the bottom rungs of the economic ladder has been in a long-term decline, even as the number of immigrants continues to climb. Other indicators associated with the underclass, such as the crime rate, have also shown improvement.
(Hat tip: Hit & Run)