Duverger's Law — a political science principle formulated by French sociologist Maurice Duverger in 1951 — predicts that plurality voting systems (first past the post) naturally converge toward two dominant parties. The mechanism is strategic voting: rational voters who prefer a third-party candidate but face the risk of a "spoiler effect" — helping their least preferred candidate win by splitting the vote with a similar candidate — tend to defect to the major party candidate they dislike least. This dynamic is visible in virtually every US presidential election where a significant third-party candidate is present: they typically receive far fewer votes than their summer polling suggested because voters "come home" to the major parties as election day approaches.

American institutional structures reinforce this at every level. To appear on the ballot in all 50 states as a presidential candidate requires collecting hundreds of thousands of valid petition signatures, a logistical and financial challenge that effectively requires either major party backing or significant independent wealth. State laws for congressional races vary but generally require third-party candidates to collect signatures at a threshold that makes it expensive and difficult. Federal campaign finance matching funds require receiving 5%+ of the vote in a previous election. The presidential debate threshold (historically set by the Commission on Presidential Debates) has been 15% in major polls — a Catch-22 since third parties rarely poll high enough to qualify without the visibility that debate inclusion would provide.

*RFK Jr. withdrew before election day; his name remained on ballots in most states.

Polling consistently shows between 40-60% of Americans saying they want a viable third party. Gallup has tracked this question since the 1990s and finds that dissatisfaction with both major parties — particularly during periods of high polarization — drives this sentiment upward. In 2023-2024, with Biden and Trump as the two candidates, 63% said they wanted a viable third party. Yet in practice, when offered an actual third-party ballot choice, most voters ultimately decline.

The gap between "want a third party" sentiment and third-party vote share reflects two separate phenomena: strategic voting (not wanting to waste a vote) and approval selection (when an actual third-party candidate is named, many voters find they don't like that specific person even if they like the concept of an alternative). RFK Jr. is the clearest recent example: he polled 15-20% in summer 2024 national surveys, but as election day approached, his support collapsed as voters engaged with his specific positions (vaccine skepticism, eclectic policy mix) rather than the abstract appeal of "not Biden or Trump."