Comparative political science has largely ignored the marked cross-national variation in Green party electoral performance. This article uses a unique aggregate dataset of 347 parliamentary elections from 32 countries over the course of 45 years to test competing theories about the causes of Green party success. The findings show that voter demand, institutions and mainstream party strategy all affect the Green vote. Green parties do well in societies with post-materialist conflicts caused by high levels of wealth or the presence of a tangible environmental dispute. The article also shows that regional decentralisation helps Green parties, but electoral systems have little effect on their vote share. Most importantly, it demonstrates that the impact of mainstream party strategy on Green electoral strength is dependent on the age of the Green party. While mainstream parties can undermine young Green parties by adopting the environmental issue, this effect is reversed once the Greens have survived a number of elections. Thus 'accommodative' mainstream party strategies eventually boost the Green vote by increasing the salience of the key Green issue. KEYWORDS Green parties; niche parties; environmentalism; party competition; post-materialism The Greens are the most enduring and cohesive new party family since the rise of the Social Democrats a century ago. From Italy to Ireland, Finland to France, Green parties have been regular participants in governing coalitions across developed democracies. And whether in government or not, many Green parties have blackmail or coalition kingmaker potential (Sartori 1976: 121-4). However, such success has not been universal. In some advanced democraciesfor example, Norway, Spain and Poland -Green parties remain fundamentally inconsequential electoral players.

When do radical parties gain support? Previous studies cite the economy and mainstream party ideological convergence as important. Responding to earlier inconsistent findings, I provide evidence for an interactive approach. Anti-system parties succeed when mainstream parties are simultaneously presiding over an ailing economy and failing to provide the diversity of political opinion for the electorate to meaningfully challenge the policies associated with this malaise, through which dissatisfaction with the status quo could otherwise be channeled. Two studies support this “crisis and convergence” model. At the aggregate-level, the anti-system vote is strongest during times of negative economic growth and widespread mainstream party ideological de-polarization. At the voter-level, the link between negative economic evaluations and radical party voting is stronger during establishment convergence and, vice versa, personal perceptions of convergence are themselves more closely related to support for these parties when the macroeconomy is sickly. Mainstream party homogeneity radicalizes the economic vote and strengthens anti-system challengers.

Why are many traditional governing parties of advanced democracies in decline? One explanation relates to public perceptions about mainstream party convergence. Voters think that the centre-left and -right are increasingly similar and this both reduces mainstream partisan loyalties and makes room for more radical challengers. Replicating and extending earlier studies, we provide evidence supporting this view. First, observational analysis of large cross-national surveys shows that people who place major parties closer together ideologically are less likely to be mainstream partisans, even when holding constant their own ideological proximity to their party. Second, a survey experiment in Germany suggests that this relationship is causal: exposure to information about policy convergence makes mainstream partisan attachments weaker. Importantly, we advance previous discussions of the convergence theory by showing that, in both our studies, ideological depolarisation is most detrimental to mainstream centre-left partisan attachments. We suggest that this is due to differing party histories.

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