Treasury Secretary Steven Mnuchin on Tuesday told a key lawmaker he would make a final decision as to whether to furnish President Trump’s tax returns by May 6, committing for the first time to a specific deadline in what has become a major Washington power struggle.

In a 10-page letter to House Ways and Means Committee Chairman Richard E. Neal (D-Mass.), Mnuchin said the Democrats’ request for Trump’s tax returns raised constitutional and privacy issues that needed to be resolved by the Justice Department before he could make a decision on how to proceed.

The letter repeatedly cast doubt on the motivations of Democrats in seeking the tax return information. Mnuchin served as Trump’s finance chairman during the 2016 presidential campaign, and the president already has signaled he does not want the information released, but multiple legal scholars have said they don’t see much precedent for Mnuchin to ignore the congressional inquiry.

Still, Treasury Department officials have taken steps to make their reluctance to turn over the records legally defensible. Tuesday’s letter contained 48 footnotes.

Neal has twice formally requested the tax returns from Internal Revenue Service Commissioner Charles P. Rettig, and the latest deadline was Tuesday at 5 p.m. Mnuchin has responded to both requests with a letter of his own, saying more time was needed while also casting doubt as to why Democrats are seeking the records in the first place.

“The Supreme Court has made clear that exposure for the sake of exposure is never a permissible purpose of congressional inquiries, and this principle is all the more important when private and legally protected tax information is at stake,” Mnuchin wrote.

House Democrats must now decide how forcefully to respond. They could wait until May 6 or move more aggressively and subpoena the records under a 1924 law that appears to give them access to virtually any tax return. Several House Democrats have accused the White House of stalling and implored their colleagues to ratchet up pressure.

One House Democrat asked his colleagues Monday to consider whether they should seek to impeach Rettig for failing to comply with their legal demands.

The White House has not offered any legal precedent for so far refusing to turn over the records. Instead, they have said that Trump is not releasing his tax returns because they are being audited.

“There is nothing nefarious there at all,” White House spokesman Hogan Gidley said Tuesday on Fox News. “This was litigated in 2016, and [Democrats] are going to keep pushing.”

Trump has not provided any details of the audit, however, and Trump’s former lawyer — Michael Cohen — has said he doesn’t believe the records are actually under audit.

Last week, Neal sent Rettig a pointed request for six years of Trump’s personal and business tax returns, giving him the new Tuesday deadline. The IRS is a division of the Treasury Department, and Mnuchin has said he has taken charge of the administration’s response.

Mnuchin’s resistance so far is part of a broader Trump administration effort to block myriad congressional investigations into the White House and Trump’s business background. The White House instructed a former personnel security director not to show up for questioning before the House Oversight and Reform Committee on Tuesday, and Monday the Trump Organization filed a lawsuit to prevent an accounting firm from complying with a committee subpoena for eight years’ worth of Trump’s financial records.

As the White House tries to mobilize opposition to the congressional investigations, some Democrats said they will ultimately prevail.

“We can squeeze these birds,” said Rep. Bill Pascrell Jr. (D-N.J.), a senior member of the House Ways and Means Committee. “We can go to court. . . . Anyone who denies us is at risk.”

During a call with other House Democrats on Monday, Rep. Jared Huffman (D-Calif.) asked other lawmakers whether they should consider beginning impeachment proceedings against Rettig for failing to comply with the law.

“I think it’s important to put this commissioner on notice that all of the legal backstops are in play here,” he said in an interview. “You don’t just get to follow Donald Trump’s orders when he is telling you to violate the law.”

House Democratic leaders were noncommittal on Huffman’s request. IRS and Treasury Department spokesmen did not respond to requests for comment.

Tuesday’s deadline was set by Neal after the IRS missed an initial April 10 cutoff date to produce six years’ worth of Trump’s personal and business returns.

Neal’s request relies on a 1924 federal law that says the IRS “shall furnish” any individual’s tax returns if they are sought by a chairman of Congress’s tax-writing committees. The law was written this way to make it nearly impossible for the White House to block disclosure of tax returns, following the Teapot Dome scandal during the Harding administration.

Neal contends that his request for Trump’s returns is part of his committee’s oversight responsibilities. IRS guidance directs the agency to audit the tax returns of presidents and vice presidents, and Neal said that he wants to determine if such an audit is taking place as appropriate, among other things.

Mnuchin has questioned the legitimacy of the request, while Trump’s personal attorney has dismissed it as a politically motivated fishing expedition and rejected Neal’s stated reasons.

In his letter Tuesday, Mnuchin said the historic nature of Neal’s request for the tax returns could play out long into the future.

“The resolution of this issue could set a precedent that will reverberate for years to come — again, regardless of which party is in power,” he wrote.

The wording of the statute does not appear to offer discretion to the commissioner of the IRS to refuse Neal’s request. However, the situation amounts to “uncharted territory” given that there has not previously been an occasion where a request for tax returns under the little-used statute has been refused, according to George K. Yin, a University of Virginia law professor who has testified on the topic before the Ways and Means Committee.

The statute itself does not specify a timeline for compliance or a penalty for an IRS commissioner or treasury secretary who fails to comply.

“Presumably if there was a refusal, and if the Congress doesn’t simply back off the request, there would be some type of conflict that would be resolved in court in some way, and then it would be up to the court to make a determination of at what point the treasury secretary has violated the law,” Yin said.

Although Neal has proceeded carefully with his request and rejected suggestions that politics are at play, other Democrats have been more open about their desire to get the returns in time to examine their contents ahead of the next presidential election. As Democrats proceed to investigate Trump and his administration on various fronts, some of them say that the contents of Trump’s tax returns could be key to their efforts.

The returns could provide insight into Trump’s entanglement with foreign governments, whether he improperly inflated or deflated the value of his assets in dealing with financial institutions, and potentially whether he benefited personally from the 2017 tax law.

“The center of this discussion better be the finances of the president of the United States. If it isn’t, it’s going to wind up as a draw, a jump ball, and that’s where we are,” said Pascrell, the New Jersey congressman, adding that the returns could show whether Trump is “fit for office.”

The issue has become a rallying cry for liberal activists, who staged a rally outside the IRS on Tax Day, April 15, to demand Trump release his returns. But Republicans have closed ranks around the president and objected to Democrats’ pursuit of the topic, particularly after special counsel Robert S. Mueller III’s report concluded without accusing Trump of collusion or obstruction of justice.

“The Democratic request for the President’s tax returns is unprecedented and sets a dangerous precedent,” Rep. Kevin Brady (R-Tex.), the top Ways and Means Republican, said in a statement this month. “The intent of Section 6103 is clear: the tax code must not to be used for political fishing expeditions.”

Both sides anticipate the issue will turn into a heated court battle, and some Democrats fear the Trump administration’s strategy will be to drag the issue out past the election.

“At some point you do have to have the House attorneys go to court for you,” said Janice Mays, a managing director at PwC who formerly served as the longtime staff director for Democrats on the Ways and Means Committee. “My personal view, I don’t know why this wouldn’t be summary judgment on the side of Richie Neal because the statute is so clear, but my instinct is the lawyers for the administration would be doing their damnedest to push against that.”

Presuming Democrats ultimately are successful in obtaining the returns, they would not become public but instead would have to be closely held by Neal as confidential taxpayer information. At some point, the Ways and Means Committee could vote to begin a process that would make the returns public.