We present a theoretical analysis of trends in overpotentials for electrocatalytic CO2 reduction based on density functional theory calculations. The analysis is based on understanding variations in the free energy of intermediates and mapping out the potential at which different elementary steps are exergonic as a measure of the catalytic activity. We study different surface structures and introduce a simple model for including the effect of adsorbate-adsorbate interactions. We find that high coverages of CO under typical reaction conditions for the more reactive transition metals affect the catalytic activity towards the CO2 reduction reaction, but the ordering of metal activities is not changed. For the hydrogen evolution reaction, a high CO coverage shifts the maximum activity towards more reactive metals than Pt.