The lawyers urged a Manhattan federal judge to deny the request made last week by opponents of the question. The opponents are seeking sanctions or contempt proceedings, saying evidence of the expert's influence was hidden or lied about.
"The motion borders on frivolous, and appears to be an attempt to reopen the evidence in this already-closed case and to drag this court into plaintiffs' eleventh-hour campaign to improperly derail the Supreme Court's resolution of the government's appeal," the lawyers wrote.
The lawyers' letter to Judge Jesse M. Furman was filed in advance of a Wednesday hearing when the judge was set to hear claims that longtime Republican gerrymandering expert Thomas Hofeller pressured the Trump administration to include the question for the first time since 1950.
Lawyers for opponents of adding the question said files found on Hofeller's computer drives after he died last year showed he contributed key wording to a Justice Department letter used to justify the question on the grounds that it was needed to protect minority voting rights.
Opponents to the question have argued that the census change is part of a wider Republican effort to restrict the political power of Democrats and Latino communities. The Justice Department filing said the opponents were trying to show that then-Acting Assistant Attorney General John Gore relied on Hofeller's private, unpublished 2015 study to draft a letter to urge the question be put on the census.
"That claim is false," they said. "Plaintiffs provide no evidence that Gore ever read, received, or was even aware of the existence of that unpublished study," the lawyers wrote. "Nor can they, because such evidence does not exist. ... There is no smoking gun here; only smoke and mirrors."
The filing and Wednesday's hearing occur just as the Supreme Court seems poised to approve adding the question. The justices heard arguments in April and will probably rule by July. In a statement released Monday, the Justice Department said the "baseless attack on the integrity of the Department and its employees is based on nothing more than fevered speculation and supposed 'new' evidence that, in reality, the plaintiffs have known for months."
It added: "The study that plaintiffs recovered from the personal belongings of a deceased private citizen played no role in the Department of Justice's letter recommending reinstatement of a citizenship question to the Census."
The Hofeller documents cited by lawyers were discovered when his estranged daughter found them on computer drives in her father's Raleigh, North Carolina, home after his death last summer. Furman 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.
Associated Press Writer Mike Balsamo in Washington contributed to this report.