Immigrants take unwanted jobs that the native-born won't touch. Educated immigrants provide high-level expertise, i.e. in electronics and health care, for lower cost. Immigrants place an extra burden on the welfare system. Immigrants generate more taxes for welfare than they use up. Immigrants put a strain on already overstrained resources.
Immigrants create new markets.
Immigrants import crime. Immigrants are, in the vast majority, law-abiding. Immigrants bring strange customs and practices with them. Strange customs and practices are as American as apple pie. Immigrants create ghettos.