The fact that incumbents win reelection at high rates is well established in the literature (Jacobson 2015). Incumbency also carries a substantial advantage at the local level (de Benedictis-Kessner 2017). What we don’t yet know definitively is whether election timing affects the odds of incumbents winning reelection.
Some worry that scheduling local contests on the same date as federal elections will give incumbents an inordinate advantage. This argument assumes that attention to presidential and Congressional elections will take attention away from local contests. If true, incumbents may win by default.
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The evidence is, however, mixed. First, the data we have suggest that voter knowledge about local political affairs is as high or higher in on-cycle elections than it is in off-cycle contests. For a deeper discussion of election timing and voter knowledge, see here. Second, research on incumbent win rates is itself mixed in its findings. One study found that on-cycle elections provide incumbents with a larger advantage than off-cycle elections do (de Benedictis-Kessner 2017). But examining data from 4000 cities and using a slightly different method, Trounstine (2012) found that higher turnout contests tend to reduce both the proportion of incumbents who run and the proportion who win.