In the late nineteenth century many native born Americans, as well as labor unions, opposed immigration. What were their concerns?

The answer is the influx of Catholics and that immigrants would take away their jobs.

In the late 19th century, many immigrants from Wester Europe came to America. On of the concerns of native-born Americans, who thought of the USA as a Protestant republic, was that the number of Catholics would increase. Further, another fear was that they would steal their jobs.