The increasing cost of political campaigns and its impact on the electoral process are issues of paramount importance in modern democracies. We propose a theory of electoral accountability in which candidates choose whether or not to commit to constituency service and whether or not to pay a campaign cost to advertise their platform. A higher campaign cost decreases voter welfare when partisan imbalance is low. However, when partisan imbalance is high, a higher campaign cost is associated with a higher expected level of constituency service. More costly campaigns can thus have a rebalancing effect that improves electoral accountability. We discuss the implications of our findings for campaign finance regulation and present empirical evidence consistent with our key predictions.