What harmed unions in the 1920s?
3 successive Pro-Business Republican Federal Governments. Relative prosperity brought wage increases without the need for Union intervention.
Union membership declined over the 1920s as relative prosperity brought more jobs and higher wages without needing to join a Union. Business made efforts to create an "Open Shop" labor environment meaning they wanted to hire anyone not just Union Members.
Republican Pro-Business Federal Governments did not work in the Union's favor although many Union members still voted Republican. Labor generally saw that the Democrats would be the most benefit to them. This paid off in the 1930's.