In the Madisonian Constitution, fragmented and overlapping institutions of authority are supposed to manage democracy’s innate rivalry, channeling competition to serve the public interest. This system of safeguards makes democracy more robust: capable of withstanding and, if need be, adapting to challenges posed by a changing problem environment. In this essay, I suggest why affective polarization poses a special threat to democratic robustness. While most scholars hypothesize that polarization’s dangers are that it leads to bimodality and extremism, I highlight a third hypothesized effect: Polarization reduces interest and information diversity in the political system. To be effective, democracy’s safeguards rely upon interest diversity, but Madison took that diversity for granted. Unique among democracy’s safeguards, federalism builds in a repository for diversity; its structure enables differences between national- and state-expressed interests, even within the same party. This diversity can be democracy hindering, as the United States’ history with racially discriminatory politics painfully makes clear, but it can also serve as a reservoir of interest and information dispersion that could protect democracy by restoring the possibility that cross-cutting cleavages emerge.