American constitutional federalism emerged from a complex matrix comprised by multiple intellectual, institutional, and experiential sources: from political theorists ranging from Machiavelli to Montesquieu and from Harrington to Hume; from colonial analogies to other dominions connected to the English realm through a common monarch, such as Ireland and seventeenth-century Scotland; and from an assortment of colonial customs, practices, and formal and informal institutional arrangements that were varied, fluctuating, contested, and in many respects underspecified. The multiplicity and diversity of these conceptual and historical inputs ensured that the nature and implementation of the federal idea would be matters of continuing political and theoretical debate.
Though the Supreme Court of the United States has played a preeminent role in the liquidation of the federal idea, its contours have been shaped by contributions from multiple centers: by state and federal legislators in decisions whether to initiate or enact legislation; by state and federal executives determining whether to approve or veto legislation with which they were presented; by state and federal judges reviewing legislation for constitutionality or determining which rules of decision to apply in cases coming before them; by statesmen and commentators motivated by combinations of high principle and immediate interest; and by the people by whom such officials were elected to or retired from office. Much of the work of constructing a working federal system has been performed incrementally, as actors in each of the branches of government, and judges most particularly, have sought in the context of particular cases or issues to find solutions to the practical problems arising from the coexistence of two semiautonomous levels of government within a single territory. Though the subjects addressed by this accumulative process have varied from generation to generation, many of the themes and tensions have proven remarkably durable. Still, the fallout from two exogenous shocks to the federal system has fundamentally reoriented the trajectory of American constitutional federalism. The first was the Civil War and the Reconstruction that followed; the second was the Great Depression and the resulting New Deal, which in the domain of political economy transformed American federalism from a regime constituted by a set of judicially enforced rules into a system constituted by a collection of political values entrusted to the democratic process.
Cushman, Barry, "Federalism" (2018). Book Chapters. 35.