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Abstract Empirical evidence shows that the energy expenditure—and thus the persistence—of organisms follows a unimodal relationship with temperature and a monotonic relationship with body size. However, most studies integrating these organismal properties into species competition have emphasized only the increasing phase of the thermal response. Understanding how performance declines beyond thermal optima is equally critical if we are to anticipate the impacts of global warming on community composition. Here, we provide a modeling framework to explore the conditions under which temperatures exceeding thermal optima hinder species coexistence. We contrast our theoretical predictions with publicly available experimental data on Drosophila . Our results show that warmer conditions reduce coexistence when species’ preferred temperature ranges scale positively with body size and their maximum metabolic rates occur at optimal temperatures. When species share similar body size and thermal optima, coexistence probability peaks at the optimum but declines more sharply under warming than under cooling. Conversely, when species differ in thermal optima but have similar body size, coexistence probability is highest at temperatures closer to the lower optimum. Together, our work establishes a formal approach to understanding species losses and community reassembly under global warming. Summary Temperature affects how organisms perform and, in turn, how species compete and live together. We develop a simple way to connect these two levels. First, we describe how each species’ consumption increases up to a thermal optimum and then declines as conditions become cooler or warmer. We then place these temperature- and size-dependent consumptions into a standard competition model and summarize coexistence by the size of the set of environmental conditions that allow all species to persist (the “feasibility domain”). Two clear outcomes emerge. When species have similar body sizes and share a thermal optimum, coexistence is highest near that optimum and declines on both sides, with a steeper drop under warming than under cooling. When species have different thermal optima but similar size, coexistence peaks at an intermediate temperature between the optima, closer to the lower one. We compare these patterns with classic laboratory data on pairwise competitions among Drosophila at 19°C and 25°C and find qualitative agreement. Our approach offers a transparent link between organismal thermal traits and community assembly, helping to explain when and why warming can reduce the range of conditions that support coexistence.