How do parties decide whether to campaign on consensual or conflictual issues, given high degrees of uncertainty? Assuming that parties strategically attempt to attract voters by writing manifestos, I argue that parties use the electoral performance of other parties as heuristics to mitigate uncertainty and to systematically learn about the most promising electoral strategy. The empirical analysis employs tools from spatial econometrics and analyzes party manifestos with regards to the parties' emphasis on the environment -- a valence issue with a niche party being the unequivocal issue owner -- in 25 EU countries from 1975 to 2015. The results reveal non-trivial interdependencies among European party systems stemming from direct transnational dependencies and indirect spillover effects among electoral strategies. The analysis further identifies conscious learning rather than mere imitation or independent decision-making as the mechanism underlying the diffusion of electoral strategies across national borders. Yet, in line with saliency-based theories, electoral competition mutes the diffusion of strategies at the domestic level. Here, only the success of issue owning green parties exerts a non-linear effect on the other parties' likelihood to address environmental issues. The results have important implications for understanding the dynamics of party competition, learning and diffusion processes, and parties' strategic behavior.