WASHINGTON — Treasury Secretary Steven Mnuchin again delayed a decision on whether to turn over President Trump’s tax returns to Congress, telling House lawmakers late Tuesday that the Treasury and Justice Departments needed until May 6 to assess the legality of the “unprecedented” request.

In a letter to Representative Richard E. Neal, the Democratic chairman of the House Ways and Means Committee, Mr. Mnuchin said that the Treasury Department “cannot act upon your request unless and until it is determined to be consistent with law.” Mr. Mnuchin said the department expected to make a final decision by May 6 “after receiving the Justice Department’s legal conclusions.”

Earlier Tuesday, Hogan Gidley, a White House spokesman, signaled that Mr. Trump was girding for a protracted fight over his tax returns.

[Update: Steven Mnuchin refuses to release Trump’s tax returns to Congress.]

“As I understand it, the president is pretty clear, once he’s out of audit, he’ll think about doing it, but he is not inclined to do so at this time,” Mr. Gidley said on Fox News.