Final answer:
The political group that only admitted native born white Protestants and claimed a 5 million membership in 1923 was the Ku Klux Klan. The earlier Know-Nothing Party also shared nativist views but was significant in the mid-19th century.
Step-by-step explanation:
The political group that admitted only native born white Protestants and claimed to have 5 million members in 1923 is identified as the Ku Klux Klan. The Know-Nothing Party, beginning in the 1830s and becoming significant in the 1850s, displayed similar nativist and anti-immigrant sentiments and restrictive membership criteria but did not attain the later size of the Klan in the 1920s. The resurgence of the Ku Klux Klan during the 1920s was marked by their promotion of "Americanism," anti-immigrant stance, and their opposition to non-white and non-Protestant populations, mirroring some of the Know-Nothing Party's ideologies from the mid-19th century.
However, it is important to note that while the Know-Nothing Party had faded into obscurity by the 1920s, the Ku Klux Klan had risen with a broader appeal and more massive membership in that era, playing into fears and prejudices of the time.