Electoral gains for populist radical-right parties have sparked debates on whether mainstream parties should engage with them, often focusing on national-level politics. However, there is little causal evidence on the effects of direct local cooperations between mainstream and populist parties. This paper fills this gap by examining local cooperations in German municipal legislatures, where mainstream parties have occasionally collaborated with the populist Alternative für Deutschland (AfD). Using a synthetic difference-in-differences design, we estimate the effects of these local cooperations on electoral support for both mainstream parties and the AfD. We find preliminary evidence that local cooperations with the AfD result in electoral losses for the AfD in subsequent elections. Potential mechanisms for this effect include the undermining of the AfD's anti-establishment appeal and shifts in valence perceptions.