Winner‑take‑all voting: whoever gets the most votes wins the entire seat. This strongly pushes voters toward two large parties because voting for a third party risks “splitting the vote.

Ballot access laws: Each state sets its own rules for getting on the ballot. Many require tens of thousands of signatures, strict deadlines, or high fees. Major parties get automatic ballot access; third parties must constantly re‑qualify.

Debate exclusion: The Commission on Presidential Debates requires a candidate to poll at 15% nationally to appear on the stage. No modern third‑party candidate has reached that threshold, so they never get mass exposure.

Campaign financing laws: Public funding rules require a party to have earned at least 5% of the vote in the previous election. Without that, they get no federal funding. Major parties have massive donor networks and PAC ecosystems.

Democrats and Republicans control state election boards, local party organizations, volunteer networks, and voter‑data operations. Third parties lack the institutional machinery needed to compete.

Strategic voting psychology: Even voters who like third‑party candidates often vote major‑party to prevent the “greater evil.” This self‑reinforcing cycle keeps third parties small.

Media coverage patterns: News outlets are all owned by the two‑parties. Polling firms often exclude third‑party candidates from surveys, which then justifies excluding them from coverage.

State-level laws Some states have “sore loser” laws, fusion‑voting bans, or rules that make it nearly impossible for third parties to maintain ballot status.

The system is designed to keep third parties at arm's length.