We investigate the incumbency advantage in proportional representation (PR) systems, focusing on the special case of successor incumbents — candidates who assume office midterm when incumbents vacate their seats. Leveraging a regression discontinuity design (RDD) and a novel measure of electoral closeness, we provide empirical evidence from Swiss cantonal elections, analyzing over 196,000 candidate records from 23 cantons since the 1950s. Our findings reveal a significant incumbency advantage: elected incumbents have a 44-percentage-point higher probability of reelection, which increases to 55 percentage points for successors. However, this difference primarily reflects the higher participation rates among successors. By distinguishing between elected and successor incumbents, we identify a “successor bias” in traditional incumbency estimates, suggesting that successor entry skews standard RDD-based measurements of incumbency effects unconditional on participation. These insights contribute to a refined understanding of incumbency dynamics in open-list PR systems and underscore the importance of accounting for successors in electoral advantage studies.