Reconstitutive congressional change is a marked and enduring shift in the fundamental dimensions of a chamber: its committees, parties, rules, members, and leaders, as well as its relations with the executive, the constituency, and the other chamber. The occurrence of such change depends on the confluence of five factors: the congressional agenda, the electorate, the political parties outside Congress, the institution's vision or beliefs about what role Congress should play in government, and the existence of a group of members who spearhead reconstitution. This article elaborates on the concept and theory of reconstitutive change by applying them to the first case of congressional reconstitution, the early Senate.