As I understand it, party competition in mature democracies is supposed to converge around two major parties, one on the center-right and one on the center-left. The reason is quite straightforwardly strategic: if either party strays too far from the center, it gives the other party the opportunity to seize the center and with it a majority; and, if either party splits or faced serious competition for votes from a new upstart party, vote splitting would doom both parties on whichever side of the ledger they happen to be on to perpetual minority status.
Obviously, you can have periods where there are multiple parties competing to become one of the two stable alternatives, but these should be relatively short transitional periods, not the normal course of affairs. And obviously as well this analysis works better for some democratic systems than for others. It makes more sense for American and British-style systems than for proportional-representation systems, for example. And it works better when you don’t have a regional or sectarian or other identity-driven party throwing a wrench into the works. But it’s a sufficiently thoroughgoing assumption that whenever a political system doesn’t conform to this assumption, it’s treated as an interesting exception.
But the exceptions are starting to devour the rule.
Canada just had an election whose results might be described as “what was supposed to happen in Britain’s last election.” That is to say: the third party came in second, the NDP substantially overtaking the Liberals for the first time in history. If the assumption that two-party competition is “natural” is to hold, the Liberals should fold, leaving two major parties (Conservatives and NDP) competing for Canadian votes. But for reasons of history if nothing else, this is unlikely to happen any time soon.
But apart from history, it’s worth pointing out that if the standard assumption of two-party normalcy were true, then the NDP would never have gotten off the ground in the first place. Ditto for the Liberal Democrats in Britain, who have been affecting the outcome of British elections for over a generation. Further afield, France, in spite of having a Presidential system, has nothing resembling two-party competition. Israel has a proportional representation system, so it’s a different case, but for a long time it was functionally a two-party system, with a right-wing bloc of parties competing with a left-wing bloc for the majority of voters, and with identity-driven parties for the ultra-Orthodox and the Arab vote that were open to any coalition (in the case of the ultra-Orthodox) or systematically excluded (in the case of the Arab parties). But not anymore. For over a decade, Israel’s political system has been rocked by the emergence of centrist parties aiming to be alternatives to the right and left. These parties have never won an election, but they haven’t gone away, and neither have Likud (even when it shrunk to a tiny rump after the first Netanyahu government) nor Labor (the former center-left powerhouse now dwindled to near-irrelevance and sitting in a right-wing coalition). Should I go on and talk about Mexico? Belgium? Switzerland?
What’s the current political science take on this? Why are third parties much more durable, under a variety of political systems (though not the American), than theory would suggest?