Why does support for mainstream parties decline? A growing literature points to economic loss as a source of political resentment. We bring this explanation one step further. We posit that the local economy qualifies the role of social capital in forging systemic support. When the economy thrives, social capital buffers discontent via interpersonal interactions. When the economy declines it exacerbates discontent, leading to a diffusion of grievances. We test our “networks of grievances” hypothesis in two settings. We first test our theory in Italy, which offers individual-level information together with fine-grained municipality-level social capital data. Second, we test the mechanism underlying our theory combining survey and local administrative data across 18 European countries. The results suggest that “networks of grievances” operate as channels of political discussions with peers, converting retrospective evaluations into systemic discontent bringing non-mainstream parties into voters’ choice sets.