California has 53 congressional seats - the most of any other state. But after this federal census the state could lose at least one.
"If you lose your rep that's a real problem and we're very close to losing two," says Colin Maclay from the USC Annenberg School. He is also part of Count The Nation which is trying to get an accurate 2020 census.
If California loses seats that would lead to changes in the state's political map as well as the entire country.
According to researchers at Northwestern University, a 1% error in state census counts could result in two to four house seats allocated to another state.
In addition to losing political power we would lose money.
The same 1% error could also result in $15 billion to $20 billion in misallocated funds to states.
"In fiscal year 2017 California got about $172 billion from the census. If you just do the math and you say $172 billion divided by 40 million people that's on the average of $4,000 a person per year over a 10-year period," says Maclay.
And that could affect hundreds of programs including Social Security, transportation, housing and schools.
Maclay says the largest undercount is children under 5 years old.
"Remember if you undercount a four-year-old you miss them for Headstart, you miss them for school lunches, school breakfasts, children's health insurance and you miss them for a decade because the census results last for 10 years."
Starting this week you'll see census takers going door-to-door trying to reach those people that have not yet filled out the census. The deadline is the end of this month.