Bernie Sanders is taking on some of the most powerful economic and political elites in the world, who will do everything possible to stop him. It’s an incredibly tough task. Winning will require that he have working people’s strongest weapon on his side: unions. Without a powerful and combative labor movement, even the most progressive politician cannot get very far.

This should be a cause for concern for everyone involved in Bernie Sanders’s political revolution. With Sanders and his establishment rivals neck and neck in the polls, labor unions — most of which have not yet issued any 2020 endorsements — could determine the Democratic primary.

Unions will also be crucial in the general election, particularly in battleground states like Wisconsin, Michigan, and Pennsylvania that flipped to Trump in 2016. And if Sanders does win the White House, implementing his ambitious campaign promises will likely hinge on the emergence of a disruptive labor movement like the one that ushered in the 1930s New Deal.