We introduce a dynamic model of position-taking and advertising, in which politicians trade off policy and reelection. We estimate career concerns for US senators, and quantify how preferences for office and policy affect electoral accountability. Senators are typically willing to make signicant policy concessions for electoral gains: the median senator is willing to give up 5% of the average distance between voters and politicians for a 1% increase in the probability of reelection. There is, however, substantial heterogeneity, which covaries with observable characteristics. Given estimated voter responsiveness, accountability is low on average, but high for office-motivated senators in competitive elections.