In a highly unusual legal maneuver, the Department of Justice moved on Tuesday to replace President Trump’s private lawyers and defend him against a defamation lawsuit brought in state court by the author E. Jean Carroll, who has accused him of raping her in a Manhattan department store in the 1990s.

Lawyers for the Justice Department said in court papers that Mr. Trump was acting in his official capacity as president when he denied ever knowing Ms. Carroll.

Citing a law called the Federal Tort Claims Act, the lawyers asserted the right to take the case from Mr. Trump’s private lawyers and move the matter from state court to federal court. The tort claims act gives employees of the federal government immunity from lawsuits, though legal experts say that it has rarely, if ever, been used to protect a president.