This paper evaluates the impact of a demand response program on the day-ahead electricity market in Colombia. The paper uses a realistic approach as it considers a curtailment service provider (a load aggregator) that is able to bid in the market. That is, the paper considers the existence of a demand-bidding program (one type of demand response program) that allows curtailment service providers to submit a bidding curve based on load preferences. Numerical results show that a demand-bidding program requires market participants (generators) to change their participation strategies as demand response might affect their dispatch. Additionally, numerical results show a significant operational cost reduction that benefits the wholesale market.