Theory suggests that three factors – the importance of ideology to primary voters, costly movement due to candidate reputations and lack of competition – all contribute to candidate divergence in US congressional elections. These predictions are analysed with new data from a 2000 mail survey that asked congressional candidates to place themselves on a left–right ideological scale. The data reveal that candidates often diverge, but that the degree of candidate polarization is variable and may be explained by factors in the theory. Candidates with firm public reputations, those who face weak general election competition, and those who experience stiff primary competition are all more likely to deviate from the median voter's position. Perhaps more importantly, the locations that candidates adopt have clear effects on their vote shares.