Final answer:
Duverger's Law is the principle that explains why a winner-take-all electoral system typically results in a two-party system, as seen in the United States, by discouraging votes for third-party candidates due to the high likelihood of their loss.
Step-by-step explanation:
The Rule of Winner-Take-All and Its Impact
The rule that states "a winner-take-all electoral system generally leads to a two-party system" is referred to as Duverger's Law.
This principle suggests that electoral systems which utilize a plurality voting system and single-member districts tend to favor the emergence and dominance of two political parties. In such systems, a candidate only needs to receive more votes than any other competitor, not necessarily a majority of the votes, to win an election.
This mechanism can discourage voting for third-party candidates due to the fear of wasting votes, as these votes might inadvertently help the less preferred candidate of the two leading parties win.
In the United States, winner-take-all elections contribute to the two-party system, as the candidate with the most votes gains the seat, and third parties find it extremely difficult to win enough votes to secure representation. This is exemplified by the American system where if a third party wins a considerable share of votes but does not come in first, they end up with no representation.
Such a system contrasts with proportional representation, where seats are allocated based on the percentage of the vote. Several factors make it challenging for third parties to gain traction in the winner-take-all system, such as the difficulty in raising funds and the vast resources of the established parties.
While winner-take-all is a key factor, the two-party system in the U.S. is also rooted in history, dating back to the initial divide between Federalists and Jeffersonian Republicans. The evolution of political factions over time has predominantly seen a reshuffling of these two main ideologies rather than the proliferation of multiple parties.