Treasury Secretary Steven Mnuchin declined to meet congressional Democrats’ Wednesday deadline to provide President Donald Trump’s tax returns, asking for more time to study the legality of their request.

Trump has refused to release his returns since he ran for president, and the clash over their release became a key front in the battles between his administration and newly-empowered Democrats in the House of Representatives. The simmering political conflict may well end up in the federal courts and spill over into the 2020 election.

“The department respects congressional oversight, and we intend to review your request carefully,” Mnuchin wrote in a letter to Ways and Means Chairman Richard Neal of Massachusetts.

Neal last Wednesday sent IRS Commissioner Charles Rettig a letter formally requesting six years of Trump’s personal and business tax returns, as well as other documents associated with them, giving a week to respond. Neal on Wednesday said he received Mnuchin’s letter and “will consult with counsel and determine the appropriate response to the commissioner in the coming days.”

Although he did not take a position on whether the returns should be turned over, Mnuchin pointed out that the Ways and Means Committee, when it was under Republican control, had issued a report saying that such a request would be “an abuse of authority” and “set a dangerous precedent.”

Read More: Will We Ever See Trump’s Tax Returns—And Does It Matter?

“Given the seriousness of these issues, which bear no connection to ordinary tax administration,” Mnuchin wrote, “we have begun consultations with the Department of Justice to ensure that our response is fully consistent with the law and the Constitution.”

Trump almost immediately said he opposed the request, and one of his lawyers, William Consovoy, on Friday asked the Treasury Department and IRS to withhold the returns until the Justice Department issues an opinion on whether the request is legal.

Members of the Trump administration, including acting White House Chief of Staff Mick Mulvaney, and their allies have called the request a political attack and a violation of Trump’s privacy.

Federal law gives the Ways and Means Committee, as well as the Senate Finance Committee and the Joint Committee on Taxation, the power to request the returns of any taxpayer, although some legal scholars believe the request needs a legitimate legislative purpose.

In his request, Neal wrote that his committee wants to ensure that the IRS is following its policy of auditing every sitting president and vice president. Democrats have long wanted the returns to examine, among other things, what financial dealings Trump might have.

Senator Chuck Grassley, an Iowa Republican and the chairman of the Senate Finance Committee said in a statement that Mnuchin was “right to consult with the Department of Justice before responding more fully. House Democrats’ unprecedented request has serious implications for all Americans and requires serious, careful analysis.”

Senator Ron Wyden, an Oregon Democrat, said in a statement that “There is no excuse for Secretary Mnuchin to not meet Chairman Neal’s deadline.”

“The law is clear and consultation with the Justice Department is not required. It’s a sign of political interference that Secretary Mnuchin said he is handling this request when authority has been delegated to the IRS,” Wyden said.

Mnuchin told lawmakers on Tuesday that the department had been in touch with White House lawyers before Neal made his request but had not personally been fully briefed on those discussions. He said, “It is our intent to follow the law.” Mnuchin has said he is concerned about the precedent the demand could set for violating taxpayer privacy.

“The general public, when they elected President Trump, made the decision without his tax returns being released,” Mnuchin said.

Trump broke with 40 years of presidential campaign tradition by declining to release his personal returns before the 2016 election. He said that he was under audit and didn’t intend to turn anything over until that process had been completed. Under questioning at a House hearing on Tuesday, Rettig said that he was unaware of any prohibition against a taxpayer releasing his or her returns while being audited.

Speaking to reporters before departing for a trip to Texas on Wednesday, the president reiterated that he would not release the returns while under audit and asserted that Americans don’t care in any case.