This article presents an experiment ( N = 127 university students) testing whether the previously found impact of conflict primes on effort‐related cardiac response is moderated by objective task difficulty. Recently, it has been shown that primed cognitive conflict increases cardiac pre‐ejection period (PEP) reactivity—an index of effort intensity—during the performance of relatively easy tasks. This effect could be attributed to conflict‐related negative affect. Consequently, as it has been shown for other types of negative affect, we expected conflict primes' effect to be task‐context dependent and thus to be moderated by objective task difficulty. In a between‐persons design, we manipulated conflict via embedded pictures of conflict‐related vs. non‐conflict‐related Stroop items in a memory task. We expected primed conflict to increase effort in a relatively easy version of the task but to lead to disengagement when task difficulty was objectively high. PEP reactivity corroborated our predictions. Rather than always increasing effort, cognitive conflict's effect on resource mobilization was context‐dependent and resulted in weak responses in a difficult task.