The Constitution requires a population count every 10 years. The numbers are primarily used to assign seats in the House of Representatives to every state. Arizona, which has eight, stands to gain one after this year's Census. The count also serves as the basis for the annual distribution of $435 billion in federal aid.
A complete count can bolster a community's political clout and send more money its way.
Sending Census workers knocking on doors in this climate in a border state where 2 million, or more than a third of the population, is Hispanic is daunting.
When the law was passed, "I was sitting here going, 'Oh great. What's going to happen?' " says Paul Fimbres, manager of the Tucson office that oversees many counties along the border.
He has a contingency plan for areas tough to penetrate: "In case that happens, we blitz," Fimbres says. "We send 15 or 20 people or teams of two or maybe three."
Sales at the Walmart Supercenter store in the border town of Nogales, about 60 miles south of Tucson, are among the highest in the company -- except lately.
The parking lot is less crowded these days because shoppers on both sides of the border are staying away, says Olivia Ainza-Kramer, president and CEO of the Nogales-Santa Cruz County Chamber of Commerce. Weekend boycotts have brought business to a standstill.
"Undocumented people in the county are fearful and suspicious," says Mary Dahl, director of community development for the county and head of its Census committee. "The timing of everything seems to be bad for the Census."
Lifelong Nogales residents Cathy Garino, 48, and Elvia "Yolie" Carrillo, 50, knock on doors to collect Census data. So far, so good, they say. Carrillo has encountered homes where 12 people live.
"Some people will hold back because of the language barrier," Garino says. "Another one told me it's your job to come to the door ... What has helped in my case is that everyone knows me."