I'm sure it was. Unions do get some help from the government, at least here in Norway we do not pay taxes on union dues – and the leftist parties do work very closely together with the unions – but that's today, it's certainly not always been like that - and at the end of the day, the unions are independent, as they should be.
I dunno, maybe it was easier in more honest times back in the 1800's when capitalists were more transparently evil. Also, I do think it helps that we don't have a nationally mandated minimum wage. Call me cynical.
Hot take: Progressive congresspeople should focus on ending union-busting, not on increasing minimum wage. The unions can handle minimum wage.