Today, the mechanism of the spending power drives the gears of the modern federal machine. But early nineteenth century constitutional debates demonstrate that the spending power is essentially a workaround, and a recent one at that – a tool by which Congress achieves certain political and legal ends while respecting the formal boundaries set by Article I and the Tenth Amendment. This “interbellum” period was enormously significant for American constitutional law, in particular the constellation of related doctrines concerning congressional power that we now place under the general heading of “federalism”: the spending power, the enumerated powers of Article I, and the anticommandeering principle of the Tenth Amendment. As a historical matter, political and legal actors in the early nineteenth century believed themselves to be living in what this Article argues was a long founding moment, in which the fundamental terms of the federal-state relationship were still open to debate. Constitutional scholars have mistakenly overlooked the constitutional creativity of the period. As a normative matter, I argue for an approach to millennial constitutional interpretation that recognizes the ever-changing nature of the landscape of constitutional permissibility, and that offers documentary evidence of the precise contours of that change. Studying the evolution of the spending power over time, especially where the text itself remains constant, demonstrates that ideas about federal structure are not fixed. Therefore, constitutional federalism itself is not fixed – a particularly important insight in an area of constitutional doctrine that is dominated by originalist approaches.