Politicians regularly bargain with colleagues and other actors. Bargaining dynamics are central to theories of legislative politics and representative democracy, bearing directly on the substance and success of legislation, policy, and on politicians’ careers. Yet, controlled evidence on how legislators bargain is scarce. Do they apply different strategies when engaging different actors? If so, what are they, and why? To study these questions, we field an ultimatum game bargaining experiment to 1,100 sitting politicians in Belgium, Canada, Germany, Switzerland, and the United States. We find that politicians exhibit a strong partisan bias when bargaining, a pattern that we document across all of our cases. The size of the partisan bias in bargaining is about double the size when politicians engage citizens than when they face colleagues. We discuss implications for existing models of bargaining and outline future research directions.