Single-Member District in AP US Government

The claim: one seat per district squeezes out third parties

The U.S. House is the model case. Federal law requires House members to be elected from single-member districts, so there are 435 districts and 435 winners, with nothing awarded to anyone who finishes second.

This connects to Unit 5: Political Participation, Topic 5.5: Third-Party Politics. The course asks you to explain how obstacles built into the electoral structure limit third-party and independent candidates. Winner-take-all voting in single-member districts is the main structural obstacle, and the course frames it in comparison to proportional voting, where parties get seats roughly matching their share of the vote.

The logic runs in a chain. Only first place wins, so a party that finishes a steady third everywhere gets zero seats. Voters see that and avoid "wasting" a vote on a candidate who can't win. Donors and strong candidates follow the voters to the two major parties. Political scientists call the tendency of single-member plurality systems to produce two dominant parties Duverger's Law.

Working the numbers: 15% of the vote, zero seats

Imagine a legislature with three single-member districts. In every district, Party A gets 45%, Party B gets 40%, and Party C gets 15%.

Party A wins all three seats with less than half the vote. Party B gets nothing even with 40%. Party C, backed by nearly one voter in six, also gets nothing.

Now scale it up to a legislature with 20 seats and the same overall vote shares. If every district splits the way these three did, Party A could win all 20 seats. A proportional allocation could instead give Parties A, B, and C about 9, 8, and 3 seats. Party C now holds seats and has a reason to keep running candidates. That gap between vote share and seat share is the whole argument, and it's the comparison the course wants you to explain.

Where the single-member district argument breaks down

Duverger's Law describes a tendency. The United Kingdom and Canada both elect their lower houses from single-member districts, yet parties like the Liberal Democrats, the Scottish National Party, and Canada's Bloc Québécois have won seats. The pattern is that third parties with support concentrated in one region can finish first in some districts. Support spread thinly everywhere gets shut out.

The U.S. shows the same thing at the presidential level. In 1968, George Wallace ran as a third-party candidate and carried five Deep South states because his support was regional. That happened through the Electoral College, which is also winner-take-all in most states, though it isn't the same thing as single-member districts.

Single-member districts aren't the only barrier, either. The course also stresses that major parties absorb third-party agendas into their own platforms, which pulls away the issue and the voters. A third party can lose seats and still influence policy that way. Finally, the district stays single-member even when the counting rule changes. Maine uses ranked-choice voting for congressional races, so the winner needs a majority after rankings are counted, but each district still elects one person.

Mix-ups to avoid: districts, gerrymanders, and the Electoral College

A few terms in this unit are easy to blur together:

• Single-member district vs. proportional voting. A single-member district gives its one seat to the top finisher, while proportional voting divides seats by vote share. The first tends toward two parties, the second toward multiparty systems.
• Single-member district vs. gerrymandering. The district is the structure. Gerrymandering is drawing district lines to help a party or group. Single-member district boundaries can be gerrymandered, but gerrymandering isn't limited to single-member districts. Also, single-member districts hurt third parties even when lines are drawn fairly.
• Single-member district vs. the Electoral College. A single-member district elects one legislator. The Electoral College chooses the president, and most states award all their presidential electors to the statewide winner, but Maine and Nebraska use a district-based method. An answer about House seats shouldn't lean on the Electoral College, and the reverse holds too.

Using single-member districts in multiple-choice and Concept Application answers

If a question asks which feature of the U.S. electoral system creates a structural barrier for third parties, winner-take-all voting in single-member districts is the answer to look for. You'll also see comparison questions setting winner-take-all systems against proportional representation. The right answer usually describes the cause-and-effect chain from winner-take-all rules to two-party dominance.

A Concept Application question might describe a country switching from proportional representation to single-member districts, or a third party that wins a large vote share but few seats. A strong response names the single-member, winner-take-all structure, explains that only first place earns a seat, and connects that to fewer viable parties or to voters abandoning minor-party candidates.

In Quantitative Analysis, a table or chart comparing party vote share to seat share is a natural fit. Point out the gap (a party's seat share falling far below its vote share), then link it to winner-take-all rules in single-member districts.