As parties serve in the executive, they also incur incumbency penalties known as “governing costs” — support attrition following a party’s service as a member of a governing coalition. I offer a causal mechanism to explain why winners of electoral contests become disillusioned with their parties during executive tenure: voters update their perceptions about how well the party represents their interests when it engages in political compromise that produces suboptimal policy outcomes. I test the effects of compromise and policy defection on ideological updating and party support in the context of Italy using a pre/post vignette survey experiment coupled with innovations in topic sampling methodology. I examine subject perceptions and support of their party based on examples of political compromise from among 25 salient policy issues. This work has major implications for our understanding of representative democracy. If voters are averse to inter-party cooperation, and subsequently punish their parties after learning about compromise, then this is potentially damaging to meaningful collaboration.