US had a two-party system pretty much since its inception - the two major parties changed over time, but the basic premise (that third party competitors are generally non-viable) holds.
The reason why US only has 2 parties is because its electoral system is first-past-the-post, and all legislative districts are single-member. The moment it switches to something like, say, MMP, I'm certain that the existing parties will disintegrate across factional lines.
The reason why US only has 2 parties is because its electoral system is first-past-the-post, and all legislative districts are single-member. The moment it switches to something like, say, MMP, I'm certain that the existing parties will disintegrate across factional lines.