Based on the implicit-affect-primes-effort (IAPE) model (Gendolla, 2012, 2015), the present experiment tested whether objective task difficulty moderates the previously found impact of fear and anger primes on effort-related cardiac response during an arithmetic task. We expected that fear primes would lead to stronger cardiac pre-ejection period (PEP) reactivity than anger primes in an easy task, but that anger primes would lead to a stronger PEP response than fear primes in a difficult task. Results corroborated these predictions. Moreover, there was no evidence that the affect primes induced conscious feelings that could explain the observed cardiac reactivity, suggesting that the primes had the intended implicit effect on effort mobilization. The findings contribute to the accumulating evidence in support of the IAPE model, showing that objective task difficulty is a moderator of implicit affect’s influence on effort-related cardiac response.