I was reading up on the impending Janus v. AFSCME decision and got to wondering: we're presently in a situation roughly analogous to that of the early 20th century, with powerful corporate interests arrayed against relatively weak (or at least in our case declining) labor interests. With that in mind, how did labor unions ever accrue enough power to become the powerhouse they were in the mid-20th century, when they had even longer odds than those presently faced by their modern-day descendants and had opponents more willing to use openly brutal and probably illegal tactics to prevent the formation of unions in the first place? What were the socioeconomic factors that permitted their rise to power? Conversely, what caused the gradual decline of unions leading into the 21st century?
how did labor unions ever accrue enough power to become the powerhouse they were in the mid-20th century
By using tactics that are now illegal. The landrum Griffin act and Taft Hartley act outlawed tools such as the wildcat strike (spontaneous strikes without consultation of union leadership), the secondary/solidarity strike (striking in solidarity with other workers in another business/industry), and the closed and union shop provisions (restrictions on joining a business without joining the union, the repeal of which allowed the spread of right to work laws.
and had opponents more willing to use openly brutal and probably illegal tactics to prevent the formation of unions in the first place?
People will simply be jailed/imprisoned for using the aforementioned tactics. They are outright illegal. Not as obviously violent as the Pinkerton strike busters but it's perfectly within the boundaries of the law, and just as threatening.
What were the socioeconomic factors that permitted their rise to power?
This is outside my purview, hopefully someone else chimes in with earlier history
Conversely, what caused the gradual decline of unions leading into the 21st century?
Right to work laws. Orwellian bullshit, really, it's not about the right to work, it's about the right to work for a pittance while your boss gets richer off you. Unions are legally required to represent all workers within their industry, so right to work laws (which prevent unions from requiring that workers pay union dues to work in an industry) functionally eviscerate the power of unions to accrue necessary material resources (from a dues-paying membership) while other legislation requires them to represent workers who actively refuse to contribute. Add to this the hierarchy present in most modern unions; decisions are made at the top, there's little opportunity for workers to engage with the mechanisms of control in the workplace.