Progressivism declined due to communism. Progressivism was replaced with nationalism due to the fear that people had of communism.
Progressivism is a political thought that encourages and promotes change through political and governmental action. Was started with a focus on labor issues, specifically child labor issues.
The decline of Progressivism as a US political movement came as the other major political groups adopted many of their platforms. The basis of progressivism was the advancement of both science and social awareness.
I have no Idea... :O
World War I