Do political parties influence opinion when citizens have a personal stake in policy? With an experimental design that exploits a naturally occurring, sharp variation in party cues, we study the effects of party cues during a collective bargaining conflict over the salary and work rights for public employees in Denmark. Even in this context—where the self-interest of public employees was strongly mobilized and where their party went against it—we find that party cues move opinion among partisans at least as much as in previous studies. But party cues do not lead citizens to go against their self-interest. Rather, we show that party cues temper the pursuit of self-interest among public employees by moderating the most extreme policy demands. These findings highlight an unappreciated potential of political parties to moderate—not fuel—extreme opinion.