> political systems with single-member districts and the plurality voting system, as in, for example, the United States, two main parties tend to emerge. In this case, votes for minor parties can potentially be regarded as splitting votes away from the most similar major party
If a third party grows it will either shrink again as voters realize they are splitting their vote against their biggest common opponent, or the third party replaces one of the two existing parties. Either way, you get two main parties.
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https://en.wikipedia.org/wiki/Duverger%27s_law
> political systems with single-member districts and the plurality voting system, as in, for example, the United States, two main parties tend to emerge. In this case, votes for minor parties can potentially be regarded as splitting votes away from the most similar major party
If a third party grows it will either shrink again as voters realize they are splitting their vote against their biggest common opponent, or the third party replaces one of the two existing parties. Either way, you get two main parties.