Duverger's "
law" plays a big role here, especially when it comes to Congress. Countries with single-member districts and plurality voting, like the US and UK, have a strong tendency toward two-party systems. In the US it's been easier for third parties to have an impact at the Presidential level than in Congress or state houses. Still, the most successful third-party Presidential bids have been regional in nature because of the workings of the Electoral College. Racist George Wallace won 46 Electoral Votes in 1968 with 13.5% of the national popular vote because he won five Southern states. Earlier examples include the
Populists in 1892 and the
Progressives in 1924, both of which had strong roots in the upper Midwest.
In contrast, industrialist Ross Perot ran as a third-party candidate in 1992 and won 19% of the vote, the most for any third-party candidate since Theodore Roosevelt in 1912. Perot also won exactly zero Electoral Votes because his voters were spread rather uniformly across the country.
Trump seems like a mix of Perot and Wallace in terms of his voting support. Perot did best in places like Alaska, Idaho, and Maine while Wallace's strength was in the traditional Confederacy. Both of these regions now support Trump.