Redistricting: How Maps Shape the Fight for the House

Every two years, voters elect all 435 members of the US House of Representatives. But before a single ballot is cast, another process has already shaped the contest: the drawing of district lines, known as redistricting.

What redistricting means

Each member of the House represents a geographic district. After a national census every ten years, states receive a number of seats based on their population, and then they must redraw their district boundaries so that each district has roughly the same number of people.

Who draws the maps

The rules differ from state to state.

• In many states, the state legislature draws the maps and the governor may sign or veto them.
• Some states use independent or bipartisan commissions to reduce political influence.
• Courts can review maps and, in some cases, order changes if they break state or federal law.

Why it matters

The shape of a district can influence which voters are grouped together. Lines that put more supporters of one party in a district can make that district safer for that party. Analysts call the deliberate drawing of lines for partisan advantage gerrymandering. Both major parties have faced such accusations in different states.

Safe seats and swing seats

Because of how maps are drawn and where people live, many districts lean strongly toward one party. As a result, only a limited number of districts are truly competitive. In a close battle for control of the House, those swing districts decide the outcome, and a few thousand votes can matter.

Legal limits

Federal law prohibits maps that dilute the voting power of racial or language minority groups, and courts have heard many cases about how far states can go. Legal fights over maps often continue between elections.

What to watch

When results arrive, it helps to look at the district level. A party can win a large share of votes statewide yet win fewer seats if the map favours its opponent. Redistricting does not decide any single election, but it sets the field on which every House race is played.