Canadian politics is, of course, shaped by its parliamentary system, which unifies the legislative and executive branches that are institutionally separate in the United States. But the countries share plurality-winner methods for winning seats in the national legislature (except in Louisiana, which seems to have decided that Napoleonic legal forms and a state constitution the length of a Michener novel weren't off enough).
Why, then, does Canada have a living, breathing, left-leaning (by which I mean claiming membership inthe Socialist International) third party that runs two provincial governments and seems on the edge of being a kingmaker in the next election, even without proportional representation? The roots of the Canadian NDP stretch back to the Non-Partisan League that was so vital in the upper plains states in the US and the western provinces of Canada in the early 20th century.
Canada manages to have both the NDP and the Bloc Quebecois, which votes left on many issues. A few thousand more votes in British Columbia would have made the NDP a member of a majority Liberal-NDP government; instead, the NDP merely has 19 seats in parliament. The United States has the Greens, which have few prospects for any seats in Congress, and, I memory serves, exactly one seat in any state legislature.
From my cursory reading, the NDP has a couple of advantages--strong links to big labor unions; and, unique among the larger Canadian parties, explicit ties to the provincial parties. The Greens in the united States are about as loosely confederated as a party can be and still think of itself as unified. I welcome others' thoughts.
--tim francis-wright The difference between theory and practice is that in theory they're the same.