The concept of niche parties, suggested by Meguid (2005) and Adams et al. (2006), is fundamentally an intuitive one. In (West) European party systems, some parties - such as Greens and radical-right parties, but perhaps also communist and regionalist parties - are different from their mainstream competitors - social democrats and Christian democrats.
What is much harder is to pin down why and how they are different. The original definitons by Meguid and Adams et al. concentrate mainly on programmatic differences. Adams et al. say they are non-centric parties, while Meguid says that they cross-cut existing cleavages and disregard economic issues, focussing instead on a small range of topics.
Like Meguid and Adams et al., the definition of niche parties I suggest is based on their programmatic characteristics, though my definition is closer to Meguid's. This definition is discussed in Defining and Measuring Niche Parties, Party Politics, 2012. Different approaches are presented by Meyer and Miller (2015) and by Bischof (forthcoming).
Put simply, I argue that niche parties are best defined as parties that emphasise a small range of topics more than their competitors. Following Meguid (2005, 2008) I also suggest that they should not talk too much about economic topics, since these are the core topic of most political systems, though this feature is probably not essential.
Other research emphasises the organisational and motivational differences of niche parties: they are more likely to be activist-dominated and more likely to be policy-seeking. So, the niche-mainstream distinction may simply be a useful proxy for other features of parties that influence their behaviour and success. This promising avenue of research has been pursued by, inter alia, Lehrer (2012), Pedersen (2012), Schumacher, Vis and de Vries (2013).
Finally, perhaps the core common feature of niche parties is that they introduce new issues, as suggested by Meguid. This has been picked up by De Vries and Hobolt (2012), who look at issue entrepreneurs.
Academic work using the niche party concept Adams, J., Clark, M., Ezrow, L., Glasgow, G. (2006) ‘Are Niche Parties Fundamentally Different from Mainstream Parties? The Causes and the Electoral Consequences of Western European Parties' Policy Shifts, 1976–1998’, American Journal of Political Science 50(3), 513-529. Elias, A. (2009) ‘From Protest to Power: Mapping the Ideological Evolution of Plaid Cymru and the Bloque Nacionalista Galego’, Regonal and Federal Studies 19(4), 533 - 557. Ezrow, L. (2008) ‘Research Note: On the inverse relationship between votes and proximity for niche parties’, European Journal of Political Research 47(2), 206-220. Ezrow, L. (2010) Linking Citizens and Parties: How Electoral Systems Matter for Electoral Representation. Oxford University Press, Oxford. Jensen, C.B., Spoon, J.-J. (2010) ‘Thinking locally, acting supranationally: Niche party behaviour in the European Parliament’, European Journal of Political Research 49(2), 174-201. Meguid, B. (2005) ‘Competition Between Unequals: The Role of Mainstream Party Strategy in Niche Party Success’, American Political Science Review 99(3), 347-359. Meguid, B. (2008) Party Competition Between Unequals: Strategies and Electoral Fortunes in Western Europe. Cambridge University Press. Spoon, J.-J. (2009) ‘Holding Their Own: Explaining the Persistence of Green Parties inFrance and the UK’, Party Politics 15(5), 615-634.