It marked the start of what could turn into a protacted legal battle between the president and congressional Democrats.
"It would be a gross abuse of power for the majority party to use tax returns as a weapon to attack, harass, and intimidate their political opponents. Once this Pandora’s box is opened, the ensuing tit-for-tat will do lasting damage to our nation," Trump's tax attorney, William Consovoy, wrote to Treasury General Counsel Brent McIntosh.
"The IRS should refrain from divulging the requested information until it receives a formal legal opinion from the Justice Department’s Office of Legal Counsel," Consovoy wrote.
Treasury Secretary Steven Mnuchin told lawmakers last month that the Treasury will “follow the law” and review any request.
On Wednesday, the Democratic chairman of the House Ways and Means Committee, Rep. Richard Neal of Massachusetts, sent a letter to IRS Commissioner Charles Rettig requesting six years of Trump’s personal and business tax information from 2013-2018, including individual tax returns and returns from eight businesses linked to Trump. He also asked whether any of them are or have been under audit -- and for all the information to be delivered to Capitol Hill within a week.
"This request is about policy, not politics," Neal said. The decision to request the returns was in the interest of ensuring "the accountability of our government and elected officials. To maintain trust in our democracy, the American people must be assured that their government is operating properly, as laws intend,” he said in a statement on Wednesday.
Democrats have said certain congressional commitee chairmen are fully entitled by law to obtain the returns and said they would use them to inform their investigations into allegations of corruption and wrongdoing in the Trump administration and the president’s family business.
"Nothing whatsoever, I have nothing to say about it," Trump said on a visit to the border wall in California when asked by a reporter for comment on his lawyer's letter.
But he then proceeded to attack the Democrats' efforts.
"I got elected. They elected me, and now they keep going. I'm under audit. When you're on audit, you don't do it. Other people are under audit, nobody would do it when you are going through audit, and I'm always under audit. They audit me all the time," Trump said.
"Chairman Neal cannot legally request -- and the IRS cannot legally divulge -- this information."
"Enforcement of our nation’s tax laws is entrusted to the IRS—an arm of the Executive Branch. Indeed, the IRS is already conducting its own examination. Congressional inquiries made “while the decisionmaking process is ongoing” impose the “greatest” intrusion on “the Executive Branch’s function of executing the law," Consovoy wrote.
ABC News’ Benjamin Siegel, John Parkinson and Jordyn Phelps contributed to this report.