Models of visual selective attention have suggested that the representation of specific features characterizing a target object is enhanced in the visual cortex, at the cost of competing task-irrelevant information. In psychophysical studies, however, such attentional enhancement has been shown to result in reduced perceptual sensitivity when maintained over periods of several seconds. Two experiments examined the relationship between target detection behavior and electrocortical facilitation in human visual cortex during sustained attention under competition, in near real time. Steady-state visual evoked potentials (ssVEPs) were used in a change detection paradigm, in which a stream of flickering grating stimuli containing target events was fully overlapping with distractor faces (experiment 1) or competing complex scenes (experiment 2), covering the same part of the visual field. Beamformer source localization was used to test plausibility of lower-tier visual cortex involvement in modulation of the ssVEP signal. Results of both experiments suggest that early overallocation of visual cortical resources to the attended stimulus stream is associated with rapid reduction of electrocortical facilitation and poor change detection across the entire trial. By contrast, temporally balanced dynamics in visual cortex predicted accurate change detection. Together, the present results support models of sustained selective attention that emphasize competition for resources in lower-tier visual cortex. These models can be extended by a temporal dimension, on which attentive behavior is characterized by frugal resource sharing across the viewing time.