Nigeria’s claim to regional leadership in West Africa has historically derived not only from its demographic size, military reach, and economic weight, but also from its perceived capacity to embody constitutional authority within the regional order. This domestic source of legitimacy has enabled Abuja to act as a principal security guarantor, diplomatic broker, and defender of democratic governance within ECOWAS, even where its own democratic record has remained imperfect. This article argues that the credibility crisis surrounding Nigeria’s 2023 general election has weakened this legitimizing foundation at a moment when ECOWAS is confronting profound institutional rupture, democratic recession, and security fragmentation following the withdrawal of Mali, Burkina Faso, and Niger. Anchored in regional security complex theory and security sector governance scholarship, the article advances the concept of democratic security legitimacy to capture the relationship between domestic electoral integrity and regional security authority. It shows how electoral credibility deficits constrain Nigeria’s regional leadership through three interrelated mechanisms: the loss of normative leverage in democratic enforcement, the weakening of security sector credibility, and the expansion of alternative regional alignments that dilute ECOWAS authority. The article concludes that Nigeria’s 2027 election is therefore more than a domestic contest for political power. It is a regional test of whether West Africa can still rely on Nigeria as a credible democratic anchor for constitutional security order.