Mr. Mnuchin’s turn in the spotlight comes at a delicate moment for him. One of the longest-serving members of the president’s cabinet, he has been dogged in recent weeks by questions over his financial ties to the film industry, as well as questions surrounding the Treasury Department’s removal of sanctions against three companies controlled by an influential Russian oligarch. Mr. Mnuchin has also been at the center of Mr. Trump’s growing frustration with the Federal Reserve, which he publicly blames for slowing American economic growth. Mr. Trump has aimed much of his criticism at the Fed chairman, Jerome H. Powell, whom the president installed to the top job on the recommendation of Mr. Mnuchin.
Mr. Mnuchin’s handling of the matter will be watched closely by Mr. Trump, who has kept his tax returns closely guarded despite promises as a candidate to eventually release them. Democrats are using a little-noticed provision of the tax code to request the documents, and the Trump administration has asserted that those efforts are outside the bounds of congressional authority and that the request is nothing more than political harassment. Last week, Representative Richard E. Neal, Democrat of Massachusetts, who leads the House Ways and Means Committee, requested that the I.R.S. hand over six years of Mr. Trump’s tax returns.
“The Democrats will never be satisfied, no matter what they get, how much they get, or how many pages they get,” Mr. Trump said on Monday in a Twitter post. “It will never end, but that’s the way life goes!”
Mick Mulvaney, the president’s acting chief of staff, said over the weekend that Democrats would “never” obtain the president’s taxes and called their efforts “a political hit job.”
Mr. Mnuchin suggested that an attempt to obtain tax returns for political purposes could ultimately harm both parties and give lawmakers discretion to obtain financial records of political enemies. Mr. Mnuchin noted that Representative Kevin Brady of Texas, who was the Republican chairman of the Ways and Means committee during the Obama administration, did not make such requests.
“I am sure there are many prominent Democrats who are relieved that when Kevin Brady was chairman of the committee that he didn’t request specific returns,” Mr. Mnuchin said.
As Mr. Mnuchin’s second hearing of the day drifted past 5 p.m., he erupted in frustration at Representative Maxine Waters, who oversees the House Financial Services Committee, saying he needed to go so he could attend a meeting with an official from Bahrain.