Objective This study evaluates how the leaders in the American Anti‐Slavery Society (AASS) either aided or hindered the formation of the Liberty Party, thus building on Bawn et al.’s (2012) theory of political party formation. Methods Regression models are used to study how the presence of AASS organizations affected Liberty Party voting in the early 1840s. Results The findings show that AASS organizations had little to no effect on Liberty voting in New England, where AASS leadership was opposed to the idea of a third party, but that a stronger AASS group presence helped the Liberty Party form outside New England, where leaders were more supportive of an anti‐slavery third party. Conclusion Interest groups do matter for the formation of political parties, but this is dependent on the support provided by, or the opposition put up by, group leaders.