Before 1900, many nativists were Protestant Christians who had their roots in northern and western Europe. In 1920s immigration quotas saw a huge number of immigrants coming from from southern and eastern Europe, many of whom were Catholics and Jews.
Americans were concerned about new arrivals coming from these regions as they were viewed to be less willing to become "Americanized".
Labor leaders, along with the nativists, pushed for immigration restrictions on these groups. New arrivals, often poor and alone, were willing to work for low wages. Unions saw them as a threat.
The federal government responded to nativist concern by passing laws to limit immigration. A 1921 law established a quota - an established number of immigrants to be allowed into the United States from various nations.
To learn more, refer to this study leaflet: A Modern Nation.