In a letter to four members of the Senate and House Appropriations Committees, The Census Project said it wanted to support the US Census Bureau in the Fiscal Year 2026 Commerce, Justice, Science Appropriations Bill, adding that the bureau needed the minimum amounts set out by the House Appropriations Committee to sustain current operations and prepare for the 2030 US census.

The letter warned that 2026 would be a key year in preparing for the 2030 census, with census tests due to take place in six sites nationwide, the results of which will feed into the census planning cycle.

“As we learned in the run up to the 2020 census, short-changing funding for testing at this point in the planning process introduces greater risk to a successful outcome,” the letter said.

“Adequate support for decennial census preparations now will reduce the risk of requiring unplanned, additional funding in the peak years at the end of the decade and improve the agency’s ability to conduct an inclusive, accurate count in 2030.”

The letter comes during an ongoing federal government shutdown in the US during an impasse over a funding agreement and spending plan, with the shutdown having closed government agencies over the past five weeks.

The Census Project has also warned that a policy provision, Section 605, could “adversely affect the quality and availability of census survey data – especially for small populations and areas”.

Section 605 states that no funding can be used to enforce involuntary compliance or to enquire more than twice for voluntary compliance with surveys carried out by the Census Bureau.

The open letter said that such a provision could have a “devastating impact” on the 2030 census, as well as other surveys such as the American Community Survey.

In a separate statement, Howard Fienberg, senior vice-president advocacy at the Insights Association, said: “The 2020 census demonstrated that shortchanging funding for research and planning in the middle of the decade is a fool’s errand, leading to the elimination of important tests and rehearsals that might have improved the accuracy of final data.

“Repeating those mistakes in the 2030 census cycle would be the height of stupidity.”