Every ten years, district boundaries are redrawn. Whoever controls that process can shape election outcomes for a decade without changing a single voter's mind. Both major parties draw favorable maps where they hold the power to do so.
Congressional and state legislative districts must be redrawn after each census to equalize population. Who does the drawing varies: in most states the legislature, in a growing number an independent or bipartisan commission, and in some cases courts when the political process deadlocks.
Two techniques do most of the work. Packing concentrates opposing voters into a few districts so they win those overwhelmingly and lose everywhere else. Cracking splits a bloc across many districts so it forms a majority in none. Both are measurable, and both are older than modern computing.
Mapping software has made the practice far more precise. Districts can now be drawn against detailed voter data to produce predictable outcomes across an entire decade, which is a change in degree large enough that many analysts treat it as a change in kind.
The legal landscape splits. The Supreme Court held in 2019 that partisan gerrymandering claims are not justiciable in federal court, leaving them to state courts and state constitutions. Racial gerrymandering claims remain reviewable under the Constitution and the Voting Rights Act, and the boundary between racial and partisan motive is heavily litigated because the two often correlate.
Mid-decade redistricting — redrawing maps between censuses — has become a live tactic in several states, prompting counter-redistricting elsewhere. Whether this is legitimate or an escalation is itself now contested.