District magnitude matters. In a one-seat district, finishing second earns nothing. Under proportional representation with several seats allocated by vote share, a smaller party may win representation without finishing first. Ranked-choice voting can reduce some spoiler incentives by allowing voters to rank alternatives, but it does not by itself create proportional representation; effects depend on district magnitude, transfer rules, and how voters and parties coordinate.

American major parties remain broad partly because factions can compete inside them. Primaries let an emerging movement seek nominations without first creating a new ballot-qualified organization. A successful faction can change a major party's candidates and program from within, while a separate minor party must overcome entry, strategic-voting, and seat-allocation barriers. The possibility of internal competition helps the two leading coalitions absorb new demands.

Exceptions do not disprove the tendency. Regional parties, independent candidates, cross-endorsement arrangements where permitted, and different local election systems can alter incentives. Social conflict and historical organization also matter; rules do not create opinions by themselves. Presidential election rules add another incentive for geographically broad alliances, although the precise ballot and elector rules vary. The causal chain is the lesson's center: plurality allocation penalizes smaller parties mechanically; expectations about that penalty produce psychological coordination; and major-party primaries offer movements an internal route for competition. A vote share that produces no seats shows the first effect. Supporters abandoning a favored minor candidate because they expect defeat shows the second.

Video lesson: Party Systems: Crash Course Government and Politics #41