In granting that request, the Commerce Department memo announcing reinstatement of the citizenship question in the 2020 Census said that “having citizenship data at the census block level will permit more effective enforcement of the VRA, and Secretary Ross determined that obtaining complete and accurate information to meet this legitimate government purpose outweighed the limited potential adverse impacts.”

The Monday evening announcement sparked renewed criticism from a wide spectrum of political actors, from voting-rights activists to immigration leaders to a coalition of America’s mayors. A blog from the Brennan Center for Justice said that “the move raises serious concerns that the upcoming census will be a major failure.” Following up on a February open letter asking Ross to deny the DOJ’s request, on Tuesday the United States Conference of Mayors decried the decision. Los Angeles Mayor Eric Garcetti declared that “the Constitution and our communities demand better than a mean-spirited attempt to intimidate immigrants and suppress participation in a count that means so much to our democracy.”

At play in the controversy are a number of different issues around both immigration and voting rights. The Justice Department argues that in order to properly enforce its mandate under the Voting Rights Act and avoid dilution of minority votes, it needs the most accurate count of the citizen-age voting population, which it argues can only be gained from asking people about their citizenship in the Census. Opponents of the measure point out that the Census Bureau’s annual supplement to the Census—the American Community Survey—already asks its sample of participants about citizenship, and that measure has been used for decades as the baseline for the citizen-age voting population.

What matters for the Justice Department’s purposes is the accuracy and the granularity of the data. The department argues that will be improved by using the Census instead of the ACS, which has a much smaller sample size and doesn’t allow researchers to peer into the composition of individual blocks, a level of detail that the Census allows.

But polling experts contend that the data extracted from this question will almost certainly be worse than the existing numbers. “Asking about citizenship on the Census is controversial because it is believed that such questions can suppress response among immigrant and other populations,” warned a press release from Kathleen Weldon at Cornell University’s Roper Center for Public Opinion Research. “The polling community is particularly concerned about adding a citizenship question to the Census, because survey researchers rely on accurate population statistics from the Census for weighting surveys.”

Unauthorized immigrants—and even authorized immigrants with undocumented family members—might have good reason to avoid taking this Census or to respond falsely, both of which would severely hamper the quality of data and make the DOJ’s claims moot. For starters, U.S. Immigration and Customs Enforcement is currently involved in a massive dragnet for unauthorized immigrants, enlisting hotels and utilizing convenience stores as sites for stings. Those efforts likely already employ existing data from the ACS on where non-citizens live. Providing data at the block level would allow ICE to locate unauthorized immigrants with greater ease.