On Monday, the Treasury Department announced that it would not be handing over President Trump’s tax returns to Congress. The announcement came after weeks of fretting from Treasury Secretary Steve Mnuchin about tax code procedures for turning over the forms.

Mnuchin, a close confidante of the President, wrote that he believes the request, “lacks a legitimate legislative purpose,” ultimately saying he doesn’t have the authority to disclose the returns. “As you have recognized, the committee’s request is unprecedented, and it presents serious constitutional questions, the resolution of which may have lasting consequences for all taxpayers,” Mr. Mnuchin wrote.

Mnuchin’s argument rests on the principle that Congress needs a clear legislative purpose when taking action. He asserts they have no purpose here, nullifying the legitimacy of the request. This is a surprising turn, as it has never been the job of the Treasury secretary to determine the legitimacy of a Congressional request. This is likely to be a sticking point once the case, inevitably, sees a judge. In the meantime, the Justice Department is set to release an official opinion sometime soon on the matter.

Massachusetts Democrat Richard E. Neal, who is chairman of the House Ways and Means Committee, which is responsible for tax matters, said he would “consult with counsel and determine the appropriate response.”

The President argues the request constitutes presidential harassment from Democrats. At the same time, his lawyers have sent letters to the Justice Department insisting they not release the returns.

In 2014, Trump said, “If I decide to run for office, I’ll produce my tax returns, absolutely.” In 2016, he said, “I would release my tax returns when the audit is complete, not after the election!” Today, it is unclear whether or not the President ever was under audit, or how that would affect him releasing the returns.

For now, the case absolutely forms a Constitutional crisis in some way or another, and Trump’s tax returns exist at the center of it.