How does the development of political parties impact the chances of elections becoming violent? Despite increasing recognition of the deleterious consequences of election violence, and increasing research on the causes of such violence, there is limited research on the role of party development in limiting or encouraging it. Leveraging new, comprehensive and constituency-level data on election violence in nineteenth-century England and Wales, we highlight how the initial development of political parties can, in certain political and institutional contexts, increase election violence. We focus on the impact of party slates – campaign coalitions between candidates from the same party – in the plurality double-member districts that prevailed in England and Wales during this period. We theorise that when candidates entered these local party slates, which formed a stepping stone toward nationally organised parties, election violence became a more attractive campaign tactic because its costs could be split without its electoral benefits to each slate-member being diminished. In support of this argument, we show that, controlling for national trends and constituency characteristics, the formation of party slates tended to be associated with increased pre-election violence, particularly of types likely to be the product of elite strategic direction. In further mechanism tests, we show that this relationship is unlikely to be driven by a growth in mass partisanship, and find some evidence that, consistent with our argument, violence served to reduce split-voting. Our findings highlight the contingent impact of party development on political development.