Representation in the U.S. House of Representatives and $400 billion in annual federal funds are allocated on the basis of the decennial census. Many other critical functions, such as enforcing civil-rights laws, also depend on accurate census data. For the first time ever, however, Congress has mandated that funding for a census be lower than for the previous census ten years earlier. Even though the Constitution requires the census to count everyone, this budget straitjacket will make it much tougher in 2020 to reach the hardest-to-count households. The results could be disastrous for all manner of public purposes that depend on accurate data, but especially for minority and low-income communities.

Although Congress had not yet passed a final budget for the Census Bureau at press time, the spending bill calls for cutting the president’s request by around 10 percent, much less than the 40 percent to 50 percent earlier expected. Nonetheless, the new cut follows earlier ones, and a report by the Senate Appropriations Committee says that the Census Bureau will have to spend less for the 2020 census than for the census in 2010, “with the goal of spending less than the 2000 census.”

The Census Bureau has a plan to deal with previously imposed budget constraints, but many of the changes it is planning are untested. In a dramatic departure, the bureau is going to rely on the Internet to collect about half of responses to the decennial census questionnaire. Americans will receive a card in the mail asking them to go online and respond. The Government Accountability Office, Congress’s watchdog, warns that without substantial up-front investments and preparation, we could be facing “another Healthcare.gov”-this time with about eight times the daily volume of visits. Since many people are unlikely to return if they are unable to fill out the questionnaire the first time, any system malfunction would be disastrous for data collection. Even if the census website operates smoothly, many advocates are concerned that people with low incomes and limited access to the Internet are unlikely to use it.

As a result of prior budget cuts, the bureau is also planning to reduce its use of canvassers in preparations for the census. Instead of sending canvassers to walk every street in America to compile an accurate address list, in-office staff will rely on a combination of aerial imagery, U.S. Postal Service records, and targeted canvassing. Using these methods, the bureau may not know whether they’ve missed people, particularly in nontraditional housing such as basement units or unmarked houses on tribal lands.

In addition, the bureau plans to cut the number of field staff deployed to follow up with households that don’t respond to the census by mail or online. The best way to ensure that people in marginalized communities are accurately counted is to talk to them face-to-face. The Census is planning to count some non-responding households indirectly through records in other government agencies or commercial data-a plan that raises serious concerns about data security, accuracy, and compatibility. Administrative and commercial records often do not include critical information the census gathers, such as data on race and ethnicity, or the information is inaccurate.

To pull off its planned changes in technology and methods, the Census Bureau would need substantial, sustained investments every year through 2020. Instead, Congress is moving to reduce those funds to a level that seriously threatens the bureau’s constitutional obligation to count everyone. An undercount is not merely a technical matter. In a democracy that allocates money and power on the basis of the census, an undercount is a matter of justice.

Gary D. Bass is the executive director of the Bauman Foundation and affiliated professor at Georgetown University’s McCourt School of Public Policy.