WASHINGTON — The United States census is so much more than just a head count. It is a snapshot of America that determines how congressional seats are apportioned, how state and federal dollars are distributed, where businesses choose to ship products and where they build new stores. To do all that properly, the count needs to be accurate.

The Commerce Department’s decision to restore a citizenship question to the census beginning in 2020 is prompting concerns about curtailing participation and possibly undercounting people living in the United States, particularly immigrants and minority groups who are expressing discomfort with answering questions from census workers.

Wilbur Ross, the commerce secretary, acknowledged concerns about decreased response rates in a memorandum released on Monday night. But he said asking about citizenship would enhance the results by helping calculate the percentage of the population eligible to vote.

An undercount of the population would have far-reaching implications. It could skew the data that are used to determine how many congressional representatives each state gets and their representation in state legislatures and local government bodies. It would shape how billions of dollars a year are allocated, including for schools and hospitals. It would undermine the integrity of a wide variety of economic data and other statistics that businesses, researchers and policymakers depend on to make decisions, including the numbers that underpin the forecasts for Social Security beneficiaries.