Time and again, the fate of important policies rises or falls on divisions within each of the two major political parties. Even in our polarized age, momentous changes to our healthcare system, environmental laws, and immigration regime hinge on intraparty debates. The same is true for deep constitutional questions. Consider that, when Donald Trump refused to concede the 2020 election, attention quickly turned to Republican officeholders. Would they back the president’s dangerous gambit and undermine the nation’s commitment to the peaceful transfer of power? Or refuse to go along, reinforcing this foundational norm? In one important respect, this precarious moment in constitutional history is not atypical. It reminds us that what happens within political parties matters to constitutional law at least as much as what happens between them. And yet, studies exploring parties and their constitutional significance focus exclusively on fights between Democrats and Republicans. As this Article argues, the fate of our constitutional system rests—for better, and often for worse—on whether (and how well) parties stick together. Take the conventional wisdom that Congress and the president will defend their institutional turf most vigorously when they are controlled by rival parties. This account assumes that members of the majority party in Congress can overcome their differences to check the executive. But how exactly this happens warrants close attention. Indeed, drawing on contemporary social science research, the Article argues that even as the two parties have grown apart, it remains difficult for party leaders to keep their members in line. The consequences are stark. For if members of the majority party in Congress are unable to reach accord, it is unlikely they will muster the collective strength to rebuff the executive. For these reasons, the tools that party leaders use to quell or disguise party divisions, and those their rank and file use to fight back, are crucial components of the “small-c” constitution. Given the key role party divisions play in structuring our democracy, they necessarily bear on the exercise of the judicial power under Article III. As this Article argues, better understanding how party divisions are resolved can help shed light on two central debates in contemporary public law: whether courts should update statutes when Congress can’t or won’t, and how courts should treat congressional silence when presidents act in ways not specifically prohibited by law. Attending to party divisions provides a blueprint for how we might strengthen our constitutional system so that it is sufficiently supple to respond to pressing problems, yet rigid enough to resist more ephemeral political trends. Stepping back, the Article concludes by making the case that anxiety over factionalism has gone too far. It argues that party divisions can ensure that elected officeholders—whether at the federal or state level—are responsive to majority will. They can also maintain the Constitution’s carefully crafted balance between stasis and energy. So, too, they can allow for a wider range of ideas to be discussed within each party, making it more likely that policymakers will find creative and effective solutions to difficult problems.