Answer:
The Canadian party system baffles outsiders and Canadians alike. Outsiders either cherry-pick details to suit a theory or just throw up their hands. Canadians wear the idiosyncrasy as a badge of honour; when looking for explanations they rarely look beyond the case. In The Canadian Party System: An Analytic History (UBC Press), I try to do the case justice from both a Canadian and a comparative perspective.
The system is anomalous in several ways. It defies the most powerful generalization in empirical political theory, Duverger’s Law. The law states that in a strongly majoritarian institutional context such as ours — most importantly, with votes counted within single-member districts — voters will concentrate on two and only two parties. Instead, Canada has had a multiparty system since at least the 1930s.
The system has spasms of massive volatility. Even though the dominant parties are among the oldest in the world, each has been to death’s door and back. In some provinces, the same citizens support fundamentally different party systems in provincial elections than in national ones. Although a party of labour exists (the NDP), the system’s class basis is weak. For most of the 20th century Canada’s political gap between Catholics and Protestants was as wide as any gap anywhere.
Finally, unlike most systems — and unlike all other systems with single-member districts — the Canadian system is dominated by a party of the centre. The Liberal Party of Canada manages to control the centre both on the classic left-right dimension and on Canada’s own “national” question. The anomaly of dominance from the centre is the key to understanding most of the other anomalies in the Canadian party system and yet begs explanation itself.
The place to start, then, is with the Liberal Party. Its dominance of Canada is a function of its dominance of Quebec. The Quebec electorate has been remarkably coordinated, both in concentration on one party at a time and in mobility between parties. These traits are then amplified by the electoral system. The rest of the country has rarely matched Quebec in electoral coherence, however. Quebec alone would put the Liberals halfway to a parliamentary majority.