Cross-party violence – violence between opposing partisans – is a growing concern worldwide. Understanding the predictors of the legitimization of violence against political opponents is thus becoming a vital goal. In this work, we propose two potential explanatory frameworks: affective polarization and low non-violent group efficacy. The first reflects an affective motivation to harm the out-party, while the latter reflects an instrumental motivation to deploy violence. Furthermore, while the former is frequently suggested in scholarly work as a cause for such violence, it has so far been backed only by limited empirical support. On the other hand, the latter is hardly examined in the context of interactions between opposing partisans. We tested the relationship of the two explanatory frameworks with the legitimization of cross-party violence using public opinion surveys in two real-world contexts: in 2020, in Israel (N = 500) and in the US (N = 631). Results from a regression analysis provide support for the instrumental explanation but not for the affective one. We discuss the theoretical implications of our findings for the research of affective polarization, and potential practical implications for attempts to reduce cross-party violence.