This paper studies the causal effect of electoral systems on the performance of clientelistic vs. programmatic parties. We argue that, contrary to majoritarian systems, proportional systems disfavor clientelistic parties as voters can hardly be pivotal for electing their local patron. We test this insight using data from local elections in Morocco from 2003 and 2009. We use a regression discontinuity approach exploiting the fact that the law stipulates a population threshold below which the system is majoritarian and above which it is proportional. Results show a differential causal effect of proportional systems on programmatic and clientelistic parties: Clientelistic parties halve their seats and the programmatic party doubles them when crossing the threshold of proportionality. An important caveat is that the sample size around the threshold being relatively small, some coefficients are estimated relatively imprecisely. Fixed effects estimates exploiting a change in threshold from 2003 to 2009 yield similar results.