Redistricting and the Census

The Constitution fixes the House at 435 seats and requires a census every ten years to count everyone living in the country. Those 435 seats get reapportioned among the 50 states based on each state's share of the total population. States that grew faster than the national average tend to gain seats, and states that grew slowly or lost people tend to lose them. Every state is guaranteed at least one seat, and the math used to split the rest is a fixed formula applied the same way after every census, so no one negotiates it.

Reapportionment only answers the question of how many seats a state gets, not where the lines go. Once a state knows its new seat count, its own state legislature, or in some states an independent commission, draws the actual district boundaries. This is redistricting, and it happens separately in each state the year after the census numbers come out. A state that gained a seat has to draw one more district than before; a state that lost a seat has to merge two old districts into one.

This two-step process is where Unit 2's machinery really shows up. Reapportionment is driven by a neutral national formula, but redistricting is a political act done by whoever controls the state legislature, which opens the door to gerrymandering, the drawing of lines to favor a party or protect incumbents. The census count that starts this whole chain also decides a huge amount else, from federal funding formulas to the size of each state's Electoral College vote, since that equals its House seats plus two senators.