Final answer:
In the 1920s, business owners were labor's prime enemy, but in the 1930s, it became the general public's belief in individual work ethic.
Step-by-step explanation:
In the 1920s, labor's prime enemy was the symbol of business, specifically the business owners who were hostile towards organized labor and attempted to challenge the legitimacy of unions through various tactics such as the American Plan. However, in the 1930s, the prime enemy of labor became the general public's continued belief in individual work ethic and the idea that organizing into radical collectives was un-American.