We study parties’ optimal ideological cohesion across electoral rules, when the following trade-off is present: A more heterogenous set of candidates is electorally appealing (catch-all party), yet, it serves policy-related goals less efficiently. When the rule becomes more disproportional, thus inducing a more favorable seat allocation for the winner, the first effect is amplified, incentivizing parties to be less cohesive. We provide empirical support using a unique data-set that records candidates’ ideological positions in Finnish municipal elections. Exploiting an exogenous change of electoral rule disproportionality at different population thresholds, we identify the causal effect of electoral rules on parties’ cohesion.