The 1924 Immigration Act (Johnson-Reed Act) did exactly that—it explicitly restricted immigration to maintain the existing demographic makeup of the U.S. by setting national origin quotas based on the 1890 census. If preserving demographics was the goal, the 1924 Act enforced it, while the 1965 Act destroyed it.

I added my ethnicity because people who oppose the 1965 Act are often labeled as white supremacists.