The alliance between social democratic parties and trade unions has shaped the political economies of Europe for more than a century. Yet in recent decades, these party--union ties have weakened considerably. We examine this development at the elite level: Why do social democrats no longer appoint trade unionists as ministers? We theorize that this is a consequence of three trends: (1) economic change that weakens the power of trade unions, (2) electoral change that makes union ties less electorally valuable for social democrats, or (3) ideological change that makes social democratic parties less friendly to unions' policy goals. We test these explanations on a novel dataset of around 1,200 ministerial appointments made by social democratic parties in Western Europe between 1960 and 2015. Our results mostly support the economic change argument: the decline in trade unionist appointments can be explained by rising unemployment and falling union density. Electoral and ideological change play much less of a role in weakening the elite-level linkages between social democrats and trade unions.
