The recent ascent and establishment of niche parties such as radical right and Green parties has fundamentally transformed Western European party systems. A lot of political science work has investigated which factors lead to more or less support for these parties. The literature has usually focused on contextual conditions such as structural transformations or the behavior of established parties. Much less research has been concerned with the behavior of niche parties themselves. In this paper we ask if Green parties can successfully appeal to a more mainstream electorate by broadening their issue appeals or if these strategies lead to strong trade-offs with their core constituency. We leverage a field and a survey experiment in Germany around the 2021 election to tackle this question. In the first study, we run a field experiment on Facebook in collaboration with the German Greens where the party randomizes which policy bundles they use to appeal to a broader electorate. We measure the effectiveness of these policy bundles on the election results in the 2021 Bundestag election. In the second study, we measure the effectiveness of the different policy bundles for particular groups of voters in a conjoint and vignette experiment ran on a population representative sample in Germany. Our study has important implications for the scholarship on party competition and helps us understand the rise of environmental politics in Europe.