This entry was inspired by chris'
Fight Fire with Fire, an interesting diary entry about how expanding the U.S. House of Representatives might correct the Electoral College's bias toward Republicans.
My thoughts are more about the two-party system, though.
I've seen it asserted here at dKos that winner-take-all election rules lead more or less inevitably to a two-party system. That is, without proportional representation, third (or fourth...) parties are pointless; the logic of winner-take-all will sooner or later force political competitors into two camps.
What this doesn't take into account is the Canadian experience. The province of Nova Scotia, for instance, uses winner-take-all rules, but has a functioning three-party system.
The 2003 Nova Scotia provincial election, for example, returned the following parliamentary configuration:
Party Politics Seats % of Vote
PC right 25 36.3
NDP left 15 31.0
Liberal center 12 31.4
This was not a fluke; Nova Scotia has had competitive three-party races going back three or four elections.
Canadian federal politics also uses winner-take-all rules, but supports four major parties (five until the recent PC-Alliance merger into the Conservative party).
Of special interest to Green sympathizers is the case of the New Democratic Party, or NDP, a historically social democratic party. The NDP holds about 15 seats in federal Parliament, despite playing under the same winner-take-all rules as the Green party does in the U.S.
From 1972 to 1974, in fact, the ruling Liberals under Trudeau did not have a majority of seats; they relied on the NDP for a working majority. That parliament enacted such leftist policies as campaign finance reform, the creation of a state petroleum company, and pension indexing.
What explains the greater openness of Canadian politics to third (and fourth...) parties?
In part, this may be a cultural difference. But more importantly, I think, is the fact that Canadian electoral districts, or "ridings" as they're known there, are much smaller than their equivalents in the U.S.
For instance, the U.S. population is roughly 270 million, and its House of Representatives has 435 members.
Canada's population is roughly 30 million, one-ninth that of the U.S., while its House of Commons has 301 members, about two-thirds that of its rough U.S. equivalent.
Consider this as a thought experiment. I live in Washington state's 7th U.S. congressional district, which is solidly Democratic. There are a lot of Greens in the district, but too few to ever carry it.
But Greens are concentrated in certain neighborhoods. Were the 7th district ever split in three, Greens might be able to win one of the new districts even under winner-take-all rules.
So another variable that determines the openness of a political system to third parties, aside from election rules (PR vs. winner-take-all) is the size of its electoral districts: the smaller the districts, the greater the openness to third parties.