What is the best term to describe the nativists of the early 20th century

anti-immigrants

Explanation:

Nativists are a certain group of people in the United States of America who are characterized by born in the United States, regardless of whether their parents were of US origin or not. In other words, they are native-born in the US.

Consequently, in the early twentieth century, these nativists opposed the increased immigration of people into the United States. This is from the belief that immigrants will affect their American culture and general way of life.

Hence, the best term to describe the nativists of the early 20th century is ANTI-IMMIGRANTS.