For nearly three decades prior to 1947 federation was the dominant and most plausible model for reforming Britain's Indian Empire. Federation offered a capacious framework for innovating upon the sovereign landscapes of empire, for imagining a wide array of nonnational futures, and for elaborating questions of rights and democracy. This essay examines official projects of federation in interwar India, efforts that culminated with the “Federation of India” envisioned by the 1935 Government of India Act. These projects sought to codify the Raj's uncodified, plural, and ambiguous imperial regime of sovereignty. As a result, the nearly six hundred “princely states” or “Indian States” had a major influence over the course of India's constitutional development. The 1935 Act inaugurated the most decisive phase in late colonial India's political and constitutional development by unleashing a competition over sovereignty in the subcontinent. It was in this context that a fully sovereign constituent assembly was adopted by the Indian National Congress as their fundamental demand. Federation played a decisive role in the development of republicanism in India.