Despite its highly candidate-centered electoral law, recent studies have shown that Brazilian party leaders are more powerful, and Brazilian parties are more unified, than alleged by long-dominant scholarship. Examining post-War and contemporary democracy in Brazil, governed by the same federal legislative electoral law, this article provides a controlled test of the role of leadership and electoral law in driving party unity. The combination of leadership intervention to enforce unity, increased unity, and partisan tides in contemporary Brazil, in contrast to an absence of leadership intervention, lower unity, and no partisan tides in the post-War, provides strong support for the role of the leadership in generating unity, as emphasized in the collective action theory of party organization. The findings also suggest that a general theory of variation in party unity requires examining factors that lead to variation in party leaders' incentives to enforce unity, in addition to the current emphasis on backbenchers' incentives to defy the leadership.