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Given that all countries that rely on a two-party system seem to have single-member legislative districts. I was wondering whether this in essence guarantees two party systems or whether this statement would not hold true.

This should not be considered a duplicate to What factors influence the number of political parties? because this asks which factors influence the number in general while this is very specific whether a given factor guarantees a certain result.

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It is easy to see that in theory there is no such guarantee (unless there are only two districts), as there is nothing to strictly prevent a different party winning in each district. Thus the theoretical maximum number of parties is given my the number of districts.

Of course in practice it is very unlikely that there will be more than two or three big parties.

The UK however is proof of the fact that even in practice a third party (in this case the Liberal Party) can become quite influential.

This doesn't necessarily mean that it would be as easy in the USA, as there are quite a few differences, to name one: in the USA a party would need to win whole states if it wants to be able to win any seats. For seats won by third parties in the US see the list of third party performances on wikipedia.

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    The Liberal party in the UK was once one of the two main parties, and was ousted from that position by the Labour party. There is certainly scope for third parties, and transition between them. – DJClayworth Dec 20 '12 at 17:18
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    There are also rare historical instances of a third party supplanting one of the two established parties (e.g. Whigs, R-Ds, Rs, Ds, Federalists) – user4012 Dec 20 '12 at 17:46
  • In Canada, we have three "major" parties, although until recently one of them (the NDP) was the runt of the three, and things generally alternated between the Liberal and Conservative parties. – Joe Z. Dec 21 '12 at 4:33
  • I would actually say that Canada has 4 major parties. Although the Bloc Quebecois currently only has 4 seats in the House. Prior to the last election in 2011 they had 47 seats, making them quite a substantial party. Who knows where things will go next time. I wouldn't discount them as a major party after just one bad election. The other major parties of course being the Conservatives, Liberals, and NDP. – Kibbee Feb 17 '13 at 18:35
  • in the USA a party would need to win whole states if it wants to be able to win any seats. Unless I'm misunderstanding, that statement is true of the US Senate but not the US House of Representatives nor is it true of state legislatures. – Mr. Bultitude May 12 '16 at 19:43
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In a "first-past-the-post" system, it is difficult to sustain more than two parties. In any given district, candidates are either in power, in opposition (and working towards becoming the party in power), or are "martyrs," meaning that they realize the have little chance of gaining majority status, but trudge on because of the "rightness" of their belief. This absolute assurance tends to be rather polarizing in practice, making it difficult to gain majority status.

That said, the tendency is not absolute. Bernie Sanders, a Socialist, has for years represented Vermont. It is just that the situation is biased towards two parties.

  • was Sanders facing opposition from non-socialist Democrats in general election? – user4012 Dec 20 '12 at 17:42
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    I would disagree that the lesser parties are all running without hope of eventually gaining power. I think they just take a longer view. – DJClayworth Dec 21 '12 at 2:23
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While the 'first-past-the-post system favours two parties, it certainly doesn't guarantee the marginalization of third parties. It depends on a number of other factors, of which geographic and demographic distribution is one. Some examples:

  • From approx 1850-1950 the British went from domination by the Liberal Party and Conservative Party to domination by Conservative and Labour parties.
  • More recently in the UK the Liberal party has strengthened its position.
  • In the Canadian 1993 election, the government went to fifth place in number of seats.
  • In the Canadian 2011 election the NDP becoems the official opposition for the first time, ousting the Liberal party.
  • In Canada the Bloc Quebecois has always been s significant force, although it only once rose to the status of official opposition. It got there by having huge support in one region of the country.

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