Depending on how quickly the courts choose to move on the litigation, though, that outcome could take months or years — a reality certain to frustrate liberals who are irate both at Mr. Trump’s vow to fight “all” congressional subpoenas and at the House’s thus far slow pace in bringing the case to court. Six months into Democratic control of the House, the tax returns lawsuit is its first attempt to enforce a subpoena in court.

Democrats have clamored for Mr. Trump’s returns since he burst onto the political stage, convinced they will show that he has distorted his assets and potentially defrauded the government. They have indicated they are preparing other lawsuits as well, including a potential suit to force Donald F. McGahn II, the former White House counsel, to testify despite White House claims that he and other top presidential advisers have “absolute immunity” from congressional subpoenas.

Mr. Trump was the first major presidential candidate in decades not to voluntarily release his tax returns. He has said that the returns were under audit by the I.R.S., but that does not actually preclude him from releasing them to the public.

The chairman of the Ways and Means Committee, Representative Richard E. Neal of Massachusetts, initially requested six years of Mr. Trump’s personal and business returns in early April using a provision of the federal tax code that grants the chairmen of Congress’s tax-writing committees the power to request tax information on any filer. The provision in question — Section 6103 — dates to the Teapot Dome scandal of the 1920s and says merely that the Treasury secretary “shall” furnish the requested material.

Mr. Neal, a Democrat, said he needed the returns, as well as audit information from the I.R.S., for a study of the efficacy of the agency’s mandatory presidential audit program that could potentially result in legislative changes.

Mr. Mnuchin rejected the request anyway, prompting Mr. Neal to shift tactics and counter in May by issuing subpoenas for the same material. That led to another rejection by Mr. Mnuchin. Both times, he said the requests lacked any “legitimate legislative purpose.”