Mr. Neal made the request through an obscure but frequently used provision of the federal tax code — Section 6103 — that allows Congress’s tax-writing committees to view tax information on any filer. The authority is regularly used by the committee to confidentially measure how changes to the tax code would affect different types of taxpayers. It has been used for more investigative purposes, as well, most recently in 2014, when committee Republicans obtained and publicly released tax information as part of an investigation into whether the I.R.S. discriminated against conservative entities seeking tax-exempt status.

But there is little precedent for using it to examine the returns of a president without his consent.

Mr. Neal gave the I.R.S. until next week to comply. If it does not, the House could go to court to try to enforce the request — a process that legal experts said could take years to sort out.



“The Supreme Court has flagged twice now that it is unclear as to whether a congressional committee can sue the president and force the executive branch to do something,” said Andy Grewal, a University of Iowa law school professor. “I think we are a very long way from getting an answer over legitimate legislative purpose.”

After months of speculation, the back-and-forth was initiated on Wednesday when Mr. Neal formally requested that the I.R.S. hand over six years of Mr. Trump’s personal and business tax returns as part of what he said was a committee oversight investigation of tax law enforcement. Even if Mr. Neal does get access to the information, it would not automatically become public because the committee would have to vote to disclose anything it sees.

In making his request, Mr. Neal jettisoned issues, like possible tax fraud, that Democrats have warned that Mr. Trump may be hiding, instead saying that he wanted to examine audit procedures for a president.

Mr. Neal worked closely with House lawyers to prepare the request to hold up against legal challenge. He framed the request as part of the Ways and Means Committee’s oversight of “the extent to which the I.R.S. audits and enforces the federal tax laws against a president.” Under I.R.S. policy, the personal tax returns of presidents and vice presidents are supposed to be automatically audited each year.