From 2010 to 2018, the state of California added about 2.3 million people according to new estimates released by the US Census. Its population, already the largest in the US, to close to 39.6 million. And California’s population increase of 6.2% has been greater than the 6.0% increase across the country.

Yet, due to the US’s complicated rules for giving states representation in Congress, California might soon lose one of its seats in the House of Representatives. Losing that seat would also mean the loss of a vote in the Electoral College, which determines the winner of presidential elections.

Since 1913, the number of representatives in the lower chamber of Congress has been capped at 435. While each state has two members in the higher chamber of the Senate, the number of its representatives to the House is determined by the size of a state’s population—though each state is automatically given one representative. (Populous California has 53 representatives, for example, and Alaska just one.)

Every 10 years, after the US government conducts its comprehensive census, the House’s representatives are reapportioned by state. For most, this doesn’t lead to a change. For those that grew much faster or slower than the country as a whole, it can mean gaining or losing power. Fast-growing Texas gained four seats after the 2010 Census as its population exploded. New York lost two seats. On average, after the 2010 Census, a House member represented about 700,000 people.

Based on 2018 population estimates, the Wall Street Journal estimated which states are likely to gain or lose House seats after the 2020 Census (paywall). West Virginia, Rhode Island, Pennsylvania, and Minnesota are among those on track to lose, while Florida, Texas and Colorado look set to make gains. (Florida is on track to pass New York as the state with the third most representation.)

The Journal analysis finds that, as things stand, California would just barely hold on to its 53rd seat. By adding just a few thousand more people, Montana could take it. It is very much in play.

The math that goes into apportionment is delicate. Inevitably, some states will get slightly more representation per person than others. The formula for apportionment tries to minimize that difference. Since California’s share of the US population has remained relatively stable, and it gained its very last representative in 2010 (pdf), a slight change to Montana’s population can make the difference. It would be the first time ever that California lost a seat.