"By narrow margin" is the most appropriate antonym for the phrase "by wide margin." The meaning of the phrase "by wide margin" is to win or lose by a significant difference, whereas the phrase "by narrow margin" means to win or lose by a small margin or difference. For example, if a team wins by a wide margin, they dominated the game, whereas if they win by a narrow margin, the outcome was uncertain until the end. Similarly, if a candidate wins the election by a wide margin, they had a decisive victory, whereas winning by a narrow margin means that the voters were closely divided.