Every ten years, it becomes necessary to re-apportion the U.S. House of Representatives because the population of the states changes. Apportionment is the first step in the redistricting process. This larger process ensures formal representation in the United States adheres to the 'one person, one vote' criteria; this principle was reaffirmed in the U.S. Supreme Court case Reynolds v. Sims (1964).
"Representatives shall be apportioned among the several states according to their respective numbers, counting the whole number of persons in each state..."— The Redistrict Network (@RedistrictNet) April 24, 2021
- U.S. Const. amend. XIV, § 2 (1868).
The U.S. Census Bureau counts the total population in each state. Once these totals are finalized, the 385 representatives not reserved to any state are divvied up. Over the course of the decade, people move, new children are born, and some people pass away. Because these human migration patterns are not uniform across states, the relative population of each state change and it become necessary to re-allocate seats.
While the total population in the U.S. grew by more than 7.4% in between 2010 and 2020, this was the slowest rate of growth since 1940 (a product of the Great Depression), and the second slowest of all time. Of course, population growth was not uniform across states. Idaho and Utah grew by more than 17%, while West Virginia lost nearly one in every twenty five people either through out-migration or death minus births.
The House of Representatives elects members in an approximately proportionate way. Early in the country's history, Congress simply increased its size after every Census so that no state would ever lose a representative, but some states would increase their share. That changed after 1929 when Congress froze the size of the House at 435.
Simply dividing the total population by each state's population leads to remainers. To determine the precise number of members each state will recieve, the Census applies what is called the "Hill-Huntington method" of apportionment, which is a formula that creates a priority list in which the first 385 are awarded.
In a typical Census, apportionment numbers are delivered by December 31 of the year ending in 0, but delays caused by COVID-19 have led the Census to spend additional months preparing the counts. When they were finally released in April 2021, several states gained one or more members of Congress, while others decreased their counts.
Apportionment also determines the Electoral College. The Electoral College is the total Members of the House plus the total number of Senators. Apportionment has huge impacts in the relative power of states, reflected in both Congress and in the EC. It is therefore a very political process, even if the formulas are determined in advance. Lawsuits typically happen after apportionment, challenging either the method, or the population counts
We can also figure out which was the last state to recieve a seat, and which is the next state that would have recieved one. Recieving an additional seat can have a huge impact on representation. In 2010, Minnesota recieved the 385th and final seat. Just missing out was North Carolina. In 2020, Minnesota kept this seat, but with just 23 persons to spare. New York, on the other hand, would have taken Minnesota's seat had it had 89 more people -- the equivalant to the residents of one floor of a Manhatten luxury apartment building!
No matter how thorough a job the Census Bureau does, some populations will surely be missed, and others double counted. One way to compare the numbers to some baseline is to measure against the Census's own projetions. In July of 2020, the Census released the Population Estimates Programs estimates. While some states had counts that exceeded their estimates, others, like Arizona, were exceedingly lower than expected.