Political campaigns are much more attack-filled in some countries than in others. What accounts for it? One answer hinges on the country’s party system. We propose that two-party systems encourage more negativity than multiparty systems because parties in a multiparty system (1) must maintain good relationships with parties with which they may want to enter into coalition and (2) run the risk of supporters of the attacked party moving to support a third party. We test the relationship between party system and attack behaviour in New Zealand, which in 1996 changed from a single-member district, first-past-the-post system to a mixed-member proportional system. The result was a more fragmented party system, resulting in coalition and minority governments. Analysing over 250 ads and party election broadcasts aired from 1969 to 2011, we find that advertising has become more positive since 1996, suggesting that party systems affect the tone of election campaigns.