WASHINGTON ― Treasury Secretary Steve Mnuchin said Friday that the administration will not comply with a subpoena for copies of President Donald Trump’s tax returns, setting up yet another legal showdown with Democrats.

Democrats’ request “lacks a legitimate legislative purpose” and the Treasury Department is therefore “not authorized” to hand over the documents, Mnuchin said in a letter to House Ways and Means Committee Chairman Richard Neal (D-Mass.).

Neal had requested the documents under a longstanding federal law giving Congress access to private tax information. Mnuchin said he’s not authorized to produce the documents, but the law actually doesn’t authorize him to refuse.

“The law, by its terms, does not allow for discretion as to whether to comply with a request for tax returns and return information,” Neal said in a statement responding to Mnuchin’s letter.

The tax returns are just one part of a broader standoff between the Trump administration and Congress. Lawmakers have a constitutional right to oversee the executive branch, but the White House has been stonewalling demands for testimony and documents.

Democrats will probably be at the mercy of federal courts to enforce their subpoenas, although some lawmakers have talked about dusting off the legislature’s own power to force compliance through jail or fines.

Neal asked for six years of Trump’s personal and business tax returns last month and followed through with the subpoena last week after Mnuchin refused the initial request.

“I didn’t think that they would turn it over,” Neal told HuffPost on Thursday, before Mnuchin had officially defied the subpoena.

Neal declined to discuss next steps but has previously said he would sue in federal court and ask a judge to enforce the subpoena. Courts have repeatedly ordered executive branch officials to cooperate with subpoenas, but the process typically takes longer than election cycles, meaning Trump could be out of office before Democrats can win.