Issue substitution or volume expansion? How parties accommodate agenda change

How do political actors respond when an issue suddenly jumps to the top of the public agenda? While conventional theories of party behaviour predict that parties increase their attention to that issue, they tell us little about how they will do so. One approach is to increase attention to the focal issue while maintaining the messaging level on other issues (volume expansion). Alternatively, political actors can increase their attention to the focal issue while decreasing their emphasis on other issues (issue substitution). We theorize that the overall volume of communication determines which approach dominates: Parties with high communication volumes will tend towards issue substitution, whereas those with lower communication volumes will prefer volume expansion. We confirm this hypothesis using a data set covering all press releases issued by members of the Austrian parliament between 2013 and 2017—a period that includes the 2015 ‘refugee crisis’ as an agenda shock.