Mr. Ross’s decision was based on the disingenuous argument that the Department of Justice needs to know the citizenship status of residents in each census tract so it can better protect the rights of minority voters under the Voting Rights Act. Even putting aside the laughable notion that this administration cares about minority voting rights, this argument is bunkum — the Justice Department has been enforcing that law without access to such data for decades. The last census that asked people to report their citizenship status was conducted in 1950, 15 years before the Voting Rights Act became law. What’s more, the Justice Department already has access to citizenship data through the American Community Survey, which is conducted every year.

The timing of this change is highly suspect. It comes too late to be included in a field test of the 2020 census the government is conducting right now with 275,000 households in Providence County, R.I. In his memo, Mr. Ross sought to play down concerns that the citizenship question would reduce response rates by claiming there was no “empirical evidence” to back up that argument. Yet, by seeking to insert the question so late in the process, he has prevented officials from empirically testing how people will react to it.

The evidence that does exist shows that concerns that the citizenship question would curb participation are legitimate. The Census Bureau reported in a September memo that its surveyors were encountering significant resistance from immigrants about providing personal information because they feared it would not be kept confidential. “The immigrant is not going to trust the census employee when they are continuously hearing a contradicting message from the media every day threatening to deport immigrants,” one Arabic-speaking respondent told the bureau.

Even Mr. Ross acknowledged in an October House hearing that adding questions to the census reduced response rates because “the more things you ask in those forms, the less likely you are to get them in.”

This is not the first time the Trump administration has sought to compromise the integrity of the census. Last year, the bureau limited its field test to Rhode Island because, it said, it didn’t have the money for tests in Washington State and West Virginia. In his budget request to Congress, the president asked for a modest increase in census funds, far less than what most experts say is needed. (Last week, lawmakers increased the bureau’s budget by $1.34 billion, about twice what he had sought.) The administration has also undermined the bureau by failing to nominate a leader since the agency’s last permanent director left in June. And, until recently, the administration was considering as deputy director a political-science professor who has argued for partisan gerrymandering and against competitive elections.