He also noted that the general census had included a citizenship question as recently as 1950, and that the question was still asked in the American Community Survey, which the Census Bureau takes among a small segment of residents. Because the wording of the question will be taken from that survey, he stated, there was no need to give it the extensive, often yearslong testing given all other census queries.

In its statement, the scientific advisory panel said Mr. Ross’s analysis was based on “data collected in a different data collection context, in a different political climate, before anti-immigrant attitudes were as salient and consequential” as they are today. The members also challenged his decision to forgo testing the question’s wording, saying it reflected different motivations and uses in the American Community Survey — including a reference to United States territories that would appear “puzzling” on the 2020 census.

The last-minute decision to include the citizenship question prevented the bureau from putting it on its only full trial run of the head count, which began this month in Providence County, R.I. The panel urged the bureau to immediately begin testing the language of the query, saying that in the current charged atmosphere surrounding foreign-born residents, the question could lead respondents to misstate or avoid answering other questions about race and ethnicity.

The committee worried as well that whatever citizenship data the bureau collected might inadvertently compromise the confidentiality of those who provided it. Federal law bars the bureau from releasing any information that specifically identifies respondents, and the bureau’s record of keeping data private is spotless. But the only citizenship data that is currently collected, from the American Community Survey, is aggregated into large parcels called census block groups.

In contrast, citizenship data collected in 2020 will be reported down to the bureau’s smallest geographic unit, called census blocks, which can be as small as an apartment building. So while individuals’ responses to the question would remain nominally private, block data could allow others to target small areas where noncitizens are reported to live.

Senior census bureau officials at the committee’s meetings carefully avoided criticizing the decision to add the citizenship question, but at the same time admitted that they did not know how much — or even how — it would affect the head count. The bureau is polling 50,000 households and conducting 42 focus groups with minorities and other slices of the population to learn their attitudes toward the census. But “there’s a lot of uncertainty,” Mr. Jarmin said. “There’s no way of really testing what it’s going to look like.”

While community organizations and minority groups have made their fears about the question clear, said Timothy Olson, the bureau’s associate director for field operations, some census offices have also seen shows of support, including an “unheard-of” 400 job applications received in one day.

“Regardless of how you feel about it, it has elevated the awareness of 2020 to the population,” Mr. Olson said, “higher than I can recall in four decennials.”