It will only happen if the United States adopts proportional representation. And it's hard to say when such a thing will happen. Certainly you'll start to see minor parties make inroads into government, but the rise of a new party to major party status is one of those events that just pop out of nowhere.
Not necessarily true. Canada has FPTP and the NDP (which was a fourth party recently) just became Official Opposition and in Australia, they use a form of PR, but they're really just a two party state as well.
Canada and the UK (and Australia) are weird cases. In the U.S. though, you're unlikely to even have a little party like the Lib Dems that constantly decides the balance of power.
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