Our primary objective was to examine the extent to which moment-to-moment associations between preschool-aged children's behavior and maternal emotional support differed for mothers showing different levels of parasympathetic engagement. We used behavioral observations of maternal and child behavior and maternal changes in cardiac vagal tone assessed via respiratory sinus arrhythmia in 15-s intervals during a 5-min challenging puzzle task (N = 121 dyads; 65 girls, M[subscript age] = 4.42 years). Results from multilevel models showed that increases in children's defeat (e.g., frustration, task withdrawal) coupled with maternal vagal augmentation (an index of social engagement) in a given 15-s interval predicted increases in maternal support in the next interval, whereas increases in children's defeat coupled with maternal vagal withdrawal (an index of physiological arousal) in a given 15-s interval did not predict increases in maternal support. Findings suggest that vagal augmentation in mothers may operate together with fluctuations in children's negative behaviors to predict supportive parenting in real time.