The Kemp's ridley (Lepidochelys kempii) is the most endangered sea turtle species. During 1966–2017, an annual count of nests (i.e., clutches of eggs laid) has served as an annual index of Kemp's ridley nesting female abundance on the Gulf of Mexico (GoM) index beach in Tamaulipas, Mexico. This index was increasing exponentially at 19% per year in 2009, but it dropped unexpectedly by more than a third in 2010 and through 2017 remained well below levels predicted. We hypothesize that pre-2010 declining carrying capacity for the Kemp's ridley population within the GoM contributed to this nesting setback. We discuss pre-2010 factors that may have caused carrying capacity to decline, including degradation of the GoM ecosystem, the exponentially increasing Kemp's ridley population, and declining per capita availability of neritic (i.e., postpelagic) Kemp's ridley food, including natural prey and scavenged discarded bycatch from shrimp trawling. We encourage evaluations (especially those within a robust modeling framework) of this hypothesis and others put forth to explain the nesting setback to provide information needed to guide restoration of the population's progress toward recovery.