James Madison and Alexander Hamilton were two of the most important thinkers of the Founding Era, yet their political interactions—a close alliance followed by a bitter rivalry—are still poorly understood. This dissertation argues that their tumultuous relationship may be explained by how each appreciated his commitment to the project of republican nationalism—the idea that the new country should be a true union of the states founded on republican principles. While they both viewed the Constitution as a step in the right direction, they had divergent ideas of how public policy could further advance this goal. Hamilton held that a political economy of mediation, whereby the monied class received direct benefits from the government that over time would strengthen the union and thus redound to the benefit of all citizens, was consistent with republican nationalism. Madison disagreed, objecting to the partiality of Hamilton's program and worrying that it lent itself to corruption. Though Madison was wrong to infer that Hamilton was a crypto-monarchist, his critique of Hamilton's economic system was meritorious in important respects. Still, he never developed an alternative political economy to bind and strengthen the union while remaining faithful to republican principles. After the nearly calamitous War of 1812, when the country's incapacity was laid bare, he adopted much of the old Hamiltonian system—and the original problems of partiality and corruption were manifest once again. Ultimately, neither found a way to deliver the kind of public policy that was consistent with the ideal embedded in the Constitution—a program to cohere and strengthen the union while remaining faithful to republican principles.