They claim that Treasury long ago delegated the responsibility to comply with congressional demands by the heads of the respective tax-writing committees to the IRS commissioner.
"The Committee's request raises serious issues concerning the constitutional scope of Congressional investigative authority, the legitimacy of the asserted legislative objective, and the constitutional right of American citizens", Mnuchin wrote.
Congressional Republicans oppose the effort, saying it would set a unsafe precedent by politicizing the confidential tax documents of a US citizen.
Committee Chairwoman Maxine Waters didn't waste time, using her first question to ask Mnuchin if he would send the president's tax information by their Wednesday deadline. When Hillary Clinton accused him, during a 2016 debate, of not paying any federal income tax, Trump said, "that makes me smart".
The White House has repeatedly rebuffed calls to release the president's tax returns, maintaining its position that any effort to disclose them amounts to little more than partisan politics.
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.
Trump's taxes have been a source of contention since his presidential campaign, when he broke with decades of tradition by refusing to release his returns. But in recent weeks, he has added to the argument, saying publicly and privately that the American people elected him without seeing his taxes and would do so again.
Iran responds in kind to Trump's IRGC 'terrorist' designation
Tehran has warned that it has missiles with a range of up to 2,000 km (1,242 miles), putting Israel and United States military bases in the region within reach.
"It will be embarrassing if I keep this person waiting", he said, per CNN.
Mnuchin thought he could outsmart Waters, by getting testy, trying to play a mind game and then having the audacity to tell the senior top Democrat how to do her job and when to bang her gavel. If the department refused to hand them over, Democrats would likely sue. "House Democrats' unprecedented request has serious implications for all Americans and requires serious, careful analysis".
"As soon as we're ready and we have this done, he's ready and willing to meet President Xi [Jinping] and it's important for the two leaders to meet and we're hopeful we can do this quickly, but we're not going to set an arbitrary deadline", Mnuchin said.
"The law is clear", Cicilline responded. "It doesn't say 'for everybody except the president'".
Neal has adopted a methodical approach to seeking Trump's returns. But it remains to be seen if the chairman would issue a subpoena if the agency doesn't comply and what a potential court fight would look like.
"We intend to follow through with this", Neal said Wednesday, speaking before Mnuchin got back to him.
The Treasury secretary said he has not spoken to Trump or Mulvaney about the request but acknowledged that Treasury Department lawyers had discussed the issue with the White House counsel's office before the request was made.