Labor unions have historically played a pivotal role in liberal democracies, channeling political demands into organized representation and forging strong ties between blue-collar constituencies and mainstream left parties. However, in recent decades, the influence of unions in the democratic process has waned, accompanied by a fragmentation of political demands. Using novel data on regional-level union density across 15 Western European countries, we test the longstanding relationship between union density and support for mainstream left parties. Our findings reveal that higher union density is positively associated with support for these parties. Over time, declining union density has contributed to increased support for radical-right parties, primarily at the expense of mainstream right parties. Furthermore, we use survey data to trace electoral flows, shedding light on these shifting political dynamics.