This is called [Duverger's law](https://en.wikipedia.org/wiki/Duverger%27s_law https://en.wikipedia.org/wiki/Duverger%27s_law) in political science. The effect

This is called [Duverger's law](https://en.wikipedia.org/wiki/Duverger%27s_law https://en.wikipedia.org/wiki/Duverger%27s_law) in political science. The effect is increased by how relatively small the US legislature is -- there are more members of the UK House of Commons than there are US Representatives and Senators combined! This makes it more difficult for third parties to develop in regional strongholds. (The US also doesn't have the strong regionalism that bolsters third parties in Canada and the UK, for that matter.) My understanding is that the presidential system also tends to favor two-party politics. The Federalists and Democratic-Republicans formed from pro- and anti-administration factions during Washington's presidency, while the Democrats and Whigs formed in support and opposition to Jackson.