Elections are ultimately a zero-sum game, where the entry of a new party inevitably leads to electoral losses for some established parties. This article examines the causal effect of a populist challenger’s candidacy on party competition, analyzing whether it primarily mobilizes formerly abstaining voters, exacerbates the decline of mainstream parties, or attenuates the rise of other challenger parties. Leveraging a natural experiment coinciding with the 2024 European election, we study County Council elections in five German states, where the newly founded Bündnis Sahra Wagenknecht (BSW) contested in some counties but not others. Using covariate balancing propensity scores, we weight municipalities on key sociodemographic, economic, and political covariates, ensuring credible as-if randomization to estimate the average treatment effect of BSW candidacy on turnout and established party vote shares. Our robust findings, generalizable beyond the local context, reveal a significant reduction in vote shares for all established parties except the right-wing populist Alternative für Deutschland (AfD), with no evidence of increased voter mobilization due to BSW’s entry. These results have important implications for party competition dynamics and the functioning of representative democracy in the presence of multiple populist challengers.