But unlike most local officials who received bad news, Mr. Duggan reacted by engaging in a public fight with the Census Bureau and suggesting he might sue. He said the bureau, under former President Donald J. Trump, did not give on-the-ground canvassers enough time to do their work last year. The change to an online questionnaire also disadvantaged the city, he said. The pandemic did not help.
“The census is just factually inaccurate,” Mr. Duggan said in an interview, noting that he raised concerns about the process last fall, long before the numbers were published. “It was census malpractice and we’re going to get it reversed.”
Census officials declined to discuss the mayor’s specific claims, but defended their work in an unsigned statement and said local officials who thought there were errors could appeal. Any corrections would not affect the data used for political redistricting, the bureau said.
There is precedent in Detroit for census disputes paying off. After the 1990 count, Coleman A. Young, the mayor at the time, challenged the tally in court and got the bureau to acknowledge that it missed tens of thousands of residents.
Still, the latest drop in the population provided a political opening for Anthony Adams, who finished a distant second to Mr. Duggan in the low-turnout mayoral primary and who will face his fellow Democrat again in the November general election.
“We’re starting to lose our Black population in the city, and we’re losing it because the policies of this administration are harmful to the people who have been here through thick and thin,” said Mr. Adams, a lawyer who has focused his campaign on crime reduction, police reform and keeping longtime residents in the city.
Even some of Mr. Duggan’s allies were unconvinced by his census rhetoric.
Paul A. Garrison II, an urban planner and economic developer who leads the Osborn Business Association, credited Mr. Duggan with nurturing new businesses, addressing problems in neighborhoods and attracting educated newcomers to Detroit. He said he even had a Duggan campaign sign in his yard. But Mr. Garrison was not buying the claims of a massive population undercount.