The latest issue of the British Journal of Political Science has an excellent article by Professors Adams and Boydstun on "'Our Issue Positions are Strong, and Our Opponents’ Valence is Weak': An Analysis of Parties' Campaign Strategies in Ten Western European Democracies."  From the Abstract: "[W]e analyze parties' national election campaign strategies across ten Western European democracies. We argue for, and empirically substantiate, an: our issues versus their valence effect, that is, that parties' self-presentations will be more issue-based than their presentations of opponents; an extremist party issue focus effect, that is, that parties with more extreme ideologies most strongly emphasize issues over valence in their self-presentations; and a prime ministerial valence focus effect, that is, that prime ministerial parties more strongly self-present on valence. These findings have implications for election outcomes and mass–elite linkages."