Parties influence public opinion far beyond their partisans: evidence from two quasi-experiments

Are parties able to influence public opinion beyond their supporters? Much of the literature on parties and public opinion finds that parties exercise considerable sway over their own partisans. However, I argue that parties in multi-party systems have broader influence – namely, they can persuade the partisans of their ally parties ("ally partisans"). Thus, I extend a key study in the field by performing difference-in-differences analyses on two rare instances in which some Danish parties shifted their policies on salient issues (unemployment benefits and early retirement), while other parties did not. However, unlike previous research, I test the influence on ally partisans. In addition, I analyse the difference-in-differences for the original partisans while omitting ally partisans from the control groups. My study also examines two potential control groups – partisans of opposing parties and nonpartisans – and concludes that the former group is more appropriate as a control. The findings demonstrate that parties influence (1) their ally partisans considerably, (2) their own partisans to a greater extent than found in past estimates, and (3) possibly even nonpartisans. This indicates that parties exercise broader and more powerful influence over public opinion than previously expected.