The Schuman Plan to “pool” the coal and steel industries of Western Europe has been widely celebrated as the founding document of today’s European Union. An expansive historiography has developed around the plan but labor and workers are largely absent from existing accounts, even though the sectors targeted for integration, coal and steel, are traditionally understood as centers of working-class militancy and union activity in Europe. Existing literature generally considers the role coal and steel industries played as objects of the Schuman Plan negotiations but this article reverses this approach. It examines instead how labor politics in the French Nord and Pas-de-Calais and the German Ruhr, core industrial regions, influenced the positions adopted by two prominent political parties, the French Socialist and German Social Democratic parties, on the integration of European heavy industry. The empirical material combines archival research in party and national archives with findings from regional histories of the Nord/Pas-de-Calais, the Ruhr, and their local socialist party chapters, as well as from historical and sociological research on miners and industrial workers. The article analyses how intense battles between socialists and communists for the allegiance of coal and steel workers shaped the political culture of these regions after the war and culminated during a mass wave of strikes in 1947–1948. The divergent political outcomes of these battles in the Nord/Pas-de-Calais and the Ruhr, this article contends, strongly contributed to the decisions of the French Socialist Party to support and the German Social Democratic Party to oppose the Schuman Plan in 1950.