WASHINGTON — President Trump on Thursday abandoned his attempt to place a question about citizenship on the 2020 Census, and instructed the government to compile citizenship data instead from existing federal records.

Trump announced in the Rose Garden that he is giving up on modifying the census two weeks after the Supreme Court rebuked his administration over its effort to do so. Just last week, Trump had insisted that his administration “must’’ pursue that goal.

“We are not backing down on our effort to determine the citizenship status of the United States population,’’ Trump said Thursday. But rather than carry on the fight over the census, he said he was issuing an executive order instructing federal departments and agencies to provide the Census Bureau with citizenship data from their “vast’’ databases immediately.

Even that order appears merely to accelerate plans the Census Bureau had announced last year, making it less a new policy than a means of covering Trump’s retreat from the composition of the 2020 Census form.

Trump struck a sharply combative tone at the opening of his remarks, saying that his political opponents were “trying to erase the very existence of a very important word and a very important thing, citizenship.’’

“The only people who are not proud to be citizens are the ones who are fighting us all the way about the word ‘citizen,’ ’’ he added.

The Trump administration has argued that including the question on census forms is an important part of its efforts to protect the voting rights of the nation’s minority residents, but the Supreme Court rejected that justification as a “contrived’’ pretext.

Government specialists have predicted that asking the question would result in many immigrants refusing to participate in the census, leading to an undercount of about 6.5 million people. That could reduce Democratic representation when congressional districts are allocated in 2021 and affect how hundreds of billions of dollars in federal spending are distributed.

In a statement, a Justice Department spokeswoman said the department would “promptly inform the courts’’ that the government would not seek to include a citizenship question in the census.

Relying on existing federal data sources could provide a clearer picture of how many people living in the United States are citizens without distorting census participation. But some Democrats complained on Thursday that the public debate itself might have sown fear among immigrants in the country and could taint their view of the census, even if it does not include a citizenship question.

Trump said his order would apply to every agency, including the Department of Homeland Security and the Social Security Administration. The Census Bureau already has access to Social Security, food stamp, and federal prison records, all of which contain citizenship information.

Trump, citing Census Bureau projections, predicted that using previously available records, the administration could determine the citizenship of 90 percent of the population ‘‘or more.’’

‘‘Ultimately this will allow us to have a more complete count of citizens than through asking the single question alone,’’ he contended.

But it is still unclear what Trump intends to do with the citizenship information. Federal law prohibits the use of census information to identify individuals, though that restriction has been breached in the past.

At one point, Trump suggested it could help states that ‘‘may want to draw state and local legislative districts based upon the voter-eligible population.’’ That would mark a change from how districts are drawn currently, based on the entire population, and could increase Republican political power.

Following Trump to the Rose Garden podium, his attorney general, William P. Barr, contended that any administration move to modify the census would have survived legal review, but only after a lengthy process that would have jeopardized the administration’s ability to conduct the census in a timely manner.

Thursday’s announcement was an anticlimactic end to a showdown that Trump escalated, in seeming defiance of the Supreme Court’s June ruling on the census question, with a July 3 post on Twitter announcing that his administration was “absolutely moving forward, as we must, because of the importance of the answer to this question.’’

Even as he waved a white flag on substance, Trump was still firing angry rhetorical shots.

“As shocking as it may be, far-left Democrats in our country are determined to conceal the number of illegal aliens in our midst,’’ he said. “They probably know the number is far greater, much higher than anyone would have ever believed before. Maybe that’s why they fight so hard.’’

But Trump’s critics relished the moment as an example of punctured hubris. Dale Ho, the director of the American Civil Liberties Union’s Voting Rights Project, said in a statement that Trump’s “attempt to weaponize the census ends not with a bang but a whimper.’’

Others called Trump’s position a face-saving measure.

“This news conference was total propaganda,’’ said Vanita Gupta, the former head of the Justice Department’s civil rights division and the chief executive of the Leadership Conference.

“The government already has access to all of this citizenship data through administrative records, and already studies it,’’ Gupta said. “Trump just didn’t want to admit defeat.’’

Material from the Associated Press was used in this report.