"Numerous courts have recognised that elected officials act within the scope of their office or employment when speaking with the press, including with respect to personal matters," the DoJ attorneys wrote.
Carroll's lawyer, Roberta Kaplan, called their argument "shocking".
"It offends me as a lawyer and offends me even more as a citizen," she said in a statement.
Carroll, meanwhile, said the developments illustrated "that Trump will do everything possible, including using the full powers of the federal government", to try to stop the case.
It will be up to a federal judge to decide whether to keep the case in federal court and to allow the US to become the defendant.
Carroll is trying to get a DNA sample from Trump to see whether it matches as-yet-unidentified male genetic material found on a dress that she says she was wearing during the alleged attack and didn't don again until a photo shoot last year.
Her suit seeks damages and a retraction of Trump's statements.
- AP