Research shows that pre-electoral coalition formations affect strategic decision making under proportional representation rules, in particular the tendency to choose small parties. This study elaborates the relationship between pre-electoral party cooperation, the number of viable parties and voter tendency to cast strategic votes, defined as a coalition-oriented vote for some other party than one’s most preferred one. The case studied is the proportional representation system of Sweden, where the number of parties has increased since the 1980s, whilst the Social Democrat’s public support has declined. Using panel data from the Swedish National Election Studies shows that the tendency to cast a strategic vote for a small party is higher when coalition signals are clear and the number of viable parties is high. The tendency to waste one’s vote on a party that do not enter parliament also increases when the prospects for government formation are more uncertain.
