Political parties’ behavior in coalition formation is commonly explained by their policy-, vote-, and office-seeking incentives. From these perspectives, the 20-year partnership of Japan's ruling conservative Liberal Democratic Party (LDP) and its pacifistic Komeito junior coalition partner is an anomalous case. The longevity, closeness, and nature of their unlikely partnership challenges core assumptions in existing theories of coalition politics. LDP–Komeito cooperation has sustained for two decades despite vastly different support bases and ideological differences on fundamental policy issues. LDP leaders also show no signs of abandoning the much smaller Komeito despite enjoying a single-party majority. We argue that the remarkable durability of this puzzling partnership results primarily from the two parties’ electoral incentives and what has effectively become codependence under Japan's mixed electoral system. Our analysis also demonstrates that being in a coalition can induce significant policy compromises, even from a much larger senior partner. Beyond theoretical implications, these phenomena yield important real-world consequences for Japanese politics: especially, a far less dominant LDP than the party's Diet seat total suggests, and Komeito's remarkable ability to punch significantly above its weight and constrain its far larger senior partner, even on the latter's major national security policy priorities.