Mainstream parties in Europe increasingly face a dilemma about whether to include populist parties in government coalitions. This has motivated a growing literature about the consequences of implementing cordons sanitaires versus tainted coalitions. This article contributes to this literature by addressing the question of whether mainstream parties tend to gain or lose vote share when they form coalition governments with populists. Building on the populism and cost of governing literatures, we explore two competing hypotheses. The “anti-populist flak” hypothesis is that voters punish mainstream parties that ally with populists because they are perceived as having betrayed democratic norms. The “big tent” hypothesis contends that—rather than being punished—mainstream parties can tap into populist support by adding them to their coalitions. Moreover, we test various mechanisms for this effect: ideological extremism of the populist party, the existence of parliamentary majorities, and intra-cabinet conflict. Using data from ParlGov and Klüver and Spoon, the preliminary results suggest that mainstream parties perform better when forming coalitions with populist junior partners.