Recent studies on strategic voting and entry in elections that combine plurality or majority and proportional representation (PR) have found candidate placement in single-member district (SMD) races to improve a party's PR performance. The primary implication of the existence of “contamination effects” is that parties have an incentive to nominate candidates in as many single-member districts as possible. Pre-electoral coordination in the majoritarian component of mixed electoral systems, however, is far from uncommon. In this article, we identify a number of institutional incentives that induce political parties to form pre-electoral alliances in spite of contamination effects. By identifying institutions that favor and hamper coordination, we seek to advance the understanding of PR-SMD interactions and to assess their implications for the design, classification, and empirical analysis of mixed electoral rules. Our statistical tests evaluate strategic entry in a diverse sample of countries.