NEW YORK — A Republican redistricting expert advocated for adding a citizenship question to the 2020 census to give an electoral edge to white people and Republicans, opponents of the move alleged in a court filing Thursday.

The filing in Manhattan federal court said a trove of newly discovered documents revealed that Thomas Hofeller, a longtime Republican gerrymandering guru, played a key role in pushing the Trump administration to include a citizenship question on the census for the first time since 1950.

Lawyers for opponents of adding the question said the files, found on Hofeller’s computer drives after he died last year, also showed that he contributed vital language to a Justice Department letter used to justify the question on the grounds that it was needed to protect minority voting rights.

In reality, the lawyers argued, the documents show the census change is part of a wider Republican effort to restrict the political power of Democrats and Latino communities.

“The new evidence reveals that Dr. Thomas Hofeller, the longtime Republican redistricting specialist, played a significant role in orchestrating the addition of the citizenship question to the 2020 Decennial Census in order to create a structural electoral advantage for, in his own words, ‘Republicans and Non-Hispanic Whites,’ and that defendants obscured his role through affirmative misrepresentations,” the filing said.

The Justice Department denied those allegations in a statement released late Thursday, calling them an “an unfortunate last-ditch effort” to derail a U.S. Supreme Court decision on the legality of adding the citizenship question.

The change, announced in spring 2018, seems poised for approval by the court, which heard arguments in April and is likely to rule by July.

It’s not yet clear if the Hofeller documents might affect the case, though the American Civil Liberties Union apprised the high court of the latest developments Thursday in a letter signed by Dale Ho, director of the group’s voting rights project and a lawyer who argued against adding the question before the top court.

States, cities and rights groups had sued in New York and elsewhere, arguing that the question would suppress the count of immigrants and strengthen congressional representation and funding for areas where mostly Republicans reside. States with large numbers of immigrants tend to vote Democratic.

Lawyers for President Donald Trump’s administration say the commerce secretary has wide discretion to design the census questionnaire.

On Thursday, lawyers for groups including the ACLU said that the files show that a Justice Department official and a transition official for Trump testified falsely by hiding Hofeller’s role in asking for the question. They asked U.S. District Judge Jesse M. Furman to issue sanctions or consider finding individuals in contempt.

Ho, of the ACLU, said documents found after Hofeller’s death last year, including a 2015 study that the redistricting expert had done, revealed the administration’s “goal was to dilute the voting power of minority communities. That’s literally the diametric opposite of what the administration has been saying all along.”

Furman gave the Justice Department until Monday to respond and set a hearing in the case for June 5.

The Justice Department said in its statement that “these eleventh-hour allegations by the plaintiffs, including an accusation of dishonesty against a senior Department of Justice official, are false.”

“That study played no role in the Department’s December 2017 request to reinstate a citizenship question to the 2020 decennial census,” it said. “The Department looks forward to responding in greater detail to these baseless accusations in its filing on Monday.”

The Hofeller documents cited by lawyers were discovered when his estranged daughter found four external computer hard drives and 18 thumb drives in her father’s Raleigh, North Carolina, home after his death last summer.

The New York Times reported that she contacted Common Cause, which had recently sued in state court to challenge North Carolina’s legislative districts, which had been drawn by Hofeller.

Furman, the federal judge, ruled in January that the question could not be included on the census, saying fewer people would respond to the census and that the process used to add it was faulty. Federal judges in California and Maryland reached similar conclusions in separate lawsuits.

Besides the citizenship question, the Supreme Court also is expected to decide within weeks, in cases from North Carolina and Maryland, whether to set limits for the first time on drawing districts for partisan advantage.

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