Gerrymandering is the practice of drawing electoral district lines to give one group an advantage. Because district maps decide which voters are grouped together, how the lines are drawn can shape who wins for a decade.
How it works: packing and cracking
Two techniques do most of the work. "Packing" concentrates the opposing party's voters into as few districts as possible, so they win those by huge margins but lose everywhere else. "Cracking" splits a bloc of voters across many districts so they can't form a majority in any of them.
Who draws the maps
District lines are redrawn after each census. In most states the legislature draws them, which lets the party in power favor itself. A growing number of states hand the job to independent or bipartisan commissions to reduce that conflict of interest.
Why it matters
Maps drawn for advantage can make many seats uncompetitive, weaken the link between how people vote statewide and how many seats each party wins, and entrench incumbents. That's why redistricting fights often end up in court.
Frequently asked questions
Is gerrymandering illegal?
Racial gerrymandering is unconstitutional, and some states bar partisan gerrymandering. But the U.S. Supreme Court has ruled that partisan gerrymandering claims are not resolvable in federal court, leaving it to states.
How is it stopped?
Through independent redistricting commissions, state courts, state constitutional limits, and ballot measures that change who controls the process.