This article analyzes the relationship between U.S. senators and their constituencies over the entire period of time that senators have been selected by direct election. Focusing on preference change within states, we identify three mechanisms that might produce responsiveness in senators’ ideological locations. We find that it is not merely the case that responsiveness is produced by party representation. Replacement of one senator with another of the same party facilitates responsiveness, too. And, even without electoral replacement, individual senators appear to adjust their ideological locations in response to changes in their electorates’ preferences. We also investigate how the mechanisms of responsiveness changed with the erosion of Democratic dominance in the South and as the parties grew stronger over time.