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How Parties Help Their Incumbents Win: Evidence from Spain

Identifying the sources of incumbency advantage has proved a dicult task. In this paper, I explore the role of parties in producing this phenomenon, a factor that has been mostly neglected by the literature. In particular, I gather data on every election to the Spanish Senate from 1977 to 2008 and exploit the multimember district system to estimate senators’ advantage over their co-partisan, non-incumbent challengers. I find a small but significant incumbency advantage, estimated to increase the probability of all incumbents being reelected by almost 25 percentage points but that of vulnerable incumbents by more than 50 percentage points. Furthermore, I find that the main source of such advantage comes from the strategic behavior of the parties, which help their more vulnerable senators get reelected by ensuring that they be placed first on the ballot.