Federal prosecutors in Manhattan told a judge Tuesday night that they are considering inserting themselves into a lawsuit filed last week by President Trump that argued he cannot be criminally investigated while in office.
The unusual court filing suggested that the president’s own Justice Department could take a position on a sweeping constitutional argument by Mr. Trump and his lawyers that has not been tested in a court.
It could also put the department in the middle of a larger dispute between Mr. Trump and the Manhattan district attorney’s office, which has issued a subpoena demanding eight years of the president’s personal and corporate returns.
Last week, Mr. Trump’s lawyers filed a lawsuit seeking to block the subpoena, arguing that a sitting president cannot be criminally investigated and releasing his tax returns would cause “irreparable harm.”
The federal prosecutors said in the three-page filing that they backed the president’s request for a temporary delay on the subpoena so that they could decide whether the government would offer an opinion on the “weighty constitutional issues” raised by the president’s lawyers.
“In view of those constitutional issues and the federal interests that they may implicate, the United States is currently considering whether to participate,” the office wrote.
The prosecutor’s office did not take a position on the merits of Mr. Trump’s argument.
It was unclear why the government chose to weigh in so late on a case in which it has had no formal role. But any potential involvement by the United States government would normally be approved, if not initiated by, the Justice Department in Washington.
The filing came the night before the judge, Victor Marrero of Federal District Court in Manhattan, was scheduled to hear arguments in the case from Mr. Trump’s lawyers and the Manhattan district attorney’s office.
A lawyer for Mr. Trump, Marc L. Mukasey, declined to comment on the government’s filing, as did a spokesman for Geoffrey S. Berman, the United States attorney in Manhattan. A spokeswoman for the Department of Justice in Washington did not respond to a request for comment.
The United States Attorney’s office said that it would inform the judge by next week whether it intended to participate in the case.
Mr. Trump’s lawsuit was filed in response to a subpoena issued late last month by the office of the Manhattan district attorney, Cyrus R. Vance Jr., to Mr. Trump’s accounting firm, Mazars USA.
The subpoena seeks the president’s personal and corporate tax returns dating to 2011. State prosecutors are investigating the role that Mr. Trump and his family business played in hush-money payments made in the run-up to the 2016 presidential election.
Both Mr. Trump and the company reimbursed Michael D. Cohen, the president’s former lawyer and fixer, for money that Mr. Cohen paid to buy the silence of Stormy Daniels, an adult film actress who said she had an affair with Mr. Trump. The president has denied the affair.
The federal prosecutors who charged Mr. Cohen said in a court filing in July that they had “effectively concluded” their investigation into possible crimes committed by the president’s company, the Trump Organization, or its executives. Neither the company nor any of its leaders were charged.
However, the office of Mr. Vance, a Democrat, is exploring whether the reimbursements violated any New York state laws.
A spokesman for Mr. Vance declined to comment on the filing by federal prosecutors.
The lawsuit was the latest effort by the president and his legal team to stymie multiple attempts to obtain copies of his tax returns, which Mr. Trump said during the 2016 campaign that he would make public but has since refused to disclose.
Mr. Trump’s lawyers have sued to block attempts by congressional Democrats and New York lawmakers to gain access to his tax returns and financial records. They also successfully challenged a California law requiring presidential primary candidates to release their tax returns.
It is an open question whether sitting presidents are immune from prosecution while in office. The Constitution does not explicitly address the issue, and the Supreme Court has never answered the question.
Federal prosecutors are barred from charging a sitting president with a federal crime because the Justice Department — in memos written during the Nixon and Clinton administrations — has decided that presidents have temporary immunity while they are in office. The memos indicate that any wrongdoing should be addressed through impeachment, not the courts.
Those memos, however, do not bind the hands of state prosecutors, and presidents have been investigated by federal authorities while in office.
In trying to block the district attorney’s subpoena, Mr. Trump’s lawyers had argued that a sitting president cannot be “investigated, indicted or otherwise subjected to criminal process.”
“Criminal investigations impose severe burdens on the president and distract him from his constitutional duties,” the lawyers said.
They also argued that the district attorney’s investigation was politically motivated, calling the subpoena “a bad-faith effort to harass the president by obtaining and exposing his confidential financial information, not a legitimate attempt to enforce New York law.”
Mr. Vance’s office, in court papers, said Mr. Trump’s claim that the prosecutors were intending to expose the president’s personal financial information was “purely speculative — and, frankly, outrageous.”
Ben Protess and Katie Benner contributed reporting.