Understanding politicians’ behavior is especially important for the Late Antebellum, when that generation may have ‘blundered’ into a bloody conflict: the US Civil War. We link the Washington Post’s database of congressional slaveholding to voting in the US House of Representatives in 1839-1861. As a summary, we examine standard voting-ideology scores and also key legislation. While slaveholding is a powerful predictor of voting, this relationship nearly vanishes if we control for representing a slave state. Relatedly, in slave states, the overwhelming majority of congressmen were slaveholders, even though the vast majority of eligible voters in that region were not. Case studies illustrate how ‘crossover’ politicians (e.g. nonslaveholders in slave states) conformed to dominant voting behavior. Our analysis favors systemic interpretations for regional differences in congressional voting, rather than this specific form of self-interest. The system brought its peculiar interests to the forefront.