Mnuchin told House Ways and Means Committee Chairman Richard Neal, who made the request a week ago, that Treasury respects congressional oversight but needs more time to review the "unprecedented" request.
Mnuchin said Neal's request raised important questions of "constitutional scope of congressional investigative authority, the legitimacy of the asserted legislative purpose, and the constitutional rights of American citizens."
He quoted Capitol Hill Republicans who called the request "Nixonian" and who warned that it could set a precedent for disclosing personal tax information for political purposes.
President Donald Trump is facing an imminent congressional deadline for his administration to provide his tax returns, but he says he "won't do it" while he's under audit by the IRS.
Trump told reporters on the White House lawn Wednesday that "I would love to give them, but I'm not going to do it while I'm under audit."
Democratic Rep. Richard Neal, a powerful House chairman, has asked the IRS to turn over six years of the president's tax returns by the end of the day.
Treasury Secretary Steven Mnuchin (mih-NOO'-shin), who supervises the IRS, said Tuesday that he's not seeking direction from the White House on whether to comply. He said the department would likely respond by Neal's deadline but didn't say whether he would provide the returns as demanded.