As to whether Congress may obtain a president’s tax returns, there is no ambiguity : Federal law empowers the chairman of the House Ways and Means Committee to submit a written request to the Treasury Department , which oversees the Internal Revenue Service, for “any return or return information.” The Treasury secretary then “shall furnish” the requested information to the committee so that it may conduct its legislative functions.

Perhaps that statute is not clear enough for Treasury Secretary Steven Mnuchin. The secretary on Monday rebuffed just such a request from Representative Richard Neal, the Democratic chairman of the Ways and Means Committee. In April, Mr. Neal requested six years’ worth of federal tax returns for President Trump and several of his companies.

The purpose of the request was not “harassment” of the president, as Mr. Trump and his defenders have branded efforts by Democratic House members to perform legitimate oversight functions. Rather, as Mr. Neal told the I.R.S., the committee “is considering legislative proposals and conducting oversight related to our federal tax laws, including, but not limited to, the extent to which the I.R.S. audits and enforces the federal tax laws against a president.”

In a letter responding to Mr. Neal on Monday, Mr. Mnuchin said he consulted with the Justice Department and determined that the chairman’s request “lacks a legitimate legislative purpose” and that the Treasury Department “may not lawfully fulfill the committee’s request.”