Marxists have had a long and complicated relationship with trade unions and the labor movement more broadly. They have been among their strongest and most steadfast supporters, but also among their most incisive critics. They have built unions up, but have also been attacked and hounded out of unions.
The core problem for Marxists is that unions occupy a paradoxical place in socialist strategy. On the one hand, they are essential for creating the collective working-class actor that is necessary to bring about socialism. On the other hand, by their very form they presuppose the continued existence of capitalism, given their primary focus on negotiating wages, benefits, and working conditions with capitalists.
So unions are a necessary but incomplete vehicle for building socialism. I explore the tensions within this contradiction, and how it has evolved since the first unions emerged in the 19th century, in a chapter in the Oxford Handbook of Karl Marx, which was published last year.
Unfortunately, that handbook is very expensive and behind a paywall (you should be able to get it if you have access to an academic library). Fortunately, Jacobin obtained permission from Oxford University Press to reprint the chapter in full. You can find it here.