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How does first-past-the-post voting affect smaller political parties?

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Map of Canada displaying the results of the 2008 federal election by province and territory, with regions shaded to indicate the leading party's share of the popular vote: blue for Conservative, red for Liberal, light blue for Bloc Quebecois, and orange for New Democratic Party. Each area is labeled with the number of seats won by each party, and a legend at the bottom summarizes national seat totals for each party. — election Canada's 2008 federal election results.

The first-past-the-post (FPTP) system, also known as the plurality or "winner-take-all" system, significantly disadvantages smaller political parties. Because this system awards a seat to the single candidate who receives the most votes in a constituency, it tends to exclude parties that may secure a substantial number of votes nationally but fail to achieve a plurality within individual districts.12

This electoral structure generally encourages the development of a two-party system, as it rewards the strongest parties disproportionately while penalizing weaker ones. Smaller parties often struggle to gain legislative representation unless their support is highly concentrated in specific geographic regions, which allows them to win individual constituencies despite a low national vote share.1234

In contrast to systems of proportional representation, which are designed to reflect the overall distribution of public support, FPTP often distorts election outcomes by exaggerating the majority of the winning party and diminishing the influence of third parties. This phenomenon, frequently linked to the work of political scientist Maurice Duverger and often referred to as "Duverger’s law," highlights the systemic pressure that plurality voting exerts against the emergence and success of smaller political entities.1435