WASHINGTON — The chairman of the House Ways and Means Committee, using a little-known provision in the federal tax code, formally requested on Wednesday that the I.R.S. hand over six years of President Trump’s personal and business tax returns, starting what is likely to be a momentous fight with his administration.
Representative Richard E. Neal, Democrat of Massachusetts, hand-delivered a two-page letter laying out the request to Charles P. Rettig, the Internal Revenue Service commissioner, ending months of speculation about when he would do so and almost certainly prompting a legal challenge from the Trump administration.
Responding to questions from reporters in the Oval Office, Mr. Trump suggested that he would fight the request because, he said, he was being audited.
“I guess when you have a name, you are audited, but until such time as I’m not under audit I would not be inclined to do that,” he said.
The move by Mr. Neal came as other panels controlled by House Democrats were flexing their muscles. The House Judiciary Committee on Wednesday morning authorized its chairman to use a subpoena to try to force the Justice Department to give Congress a full copy of the special counsel’s report and all of the underlying evidence used to reach his conclusions on Russian interference in the 2016 election.
And the chairman of the House Oversight and Reform Committee said that he would soon ask for a vote on a subpoena of his own to compel Mazars USA, an accounting firm tied to the president, to produce a decade’s worth of Mr. Trump’s financial records.
“They have told us that they will provide the information pretty much when they have a subpoena,” the chairman, Representative Elijah E. Cummings of Maryland, told reporters. “And we’ll get them a subpoena.”
Unlike the chairmen of other committees, Mr. Neal is not relying on a subpoena or standard congressional processes. Instead, he is invoking an authority enshrined in the tax code granted only to the tax-writing committees in Congress that gives the chairmen of the House Ways and Means Committee and the Senate Finance Committee the power to request tax information on any filer.
Mr. Neal gave the agency until April 10 to comply with the request, and if he receives the information, he will then confidentially review it with his committee staff.
The provision, which dates in some form to the Teapot Dome scandal of Warren G. Harding’s administration, at least on its face gives the Trump administration little room to decline a request like Mr. Neal’s. It only says that the Treasury secretary “shall” furnish the information.
“President Trump is the first president in nearly a half century to break precedent and refuse to voluntarily release his tax returns,” said Representative Dan Kildee, Democrat of Michigan and a member of the Ways and Means Committee. “The president is the only person who can sign bills into law, and the public deserves to know whether the president’s personal financial interests affect his public decision making.”
The Treasury Department and the I.R.S. did not immediately respond to requests for comment.
But Democrats anticipate that the Trump administration will object to the request and force the matter into the courts, where its adjudication could take months or longer. Though the provision — No. 6103 in the tax code — is invoked frequently by the committee, there is little precedent for using it to view the returns of a president who has not invited the scrutiny.
Republicans have vigorously argued against the request, saying that whatever justification Democrats produce will belie their true intent: to fish for information that could embarrass the president politically.
Representative Kevin Brady of Texas, the top Republican on the Ways and Means Committee, called the request “an abuse of the tax-writing committees’ statutory authority.”
“Weaponizing our nation’s tax code by targeting political foes sets a dangerous precedent and weakens Americans’ privacy rights,” Mr. Brady said in a letter to Treasury Secretary Steven Mnuchin. “As you know, by law all Americans have a fundamental right to the privacy of the personal information found in their tax returns.”
Defying modern presidential norms, Mr. Trump has refused since he became a candidate for president to release any of his tax returns. Democrats suspect the tax information could provide clues to wrongdoing by Mr. Trump, and they made getting the documents one of their top oversight priorities when they reclaimed control of the House in January.
Mr. Neal said he was making the request as part of his 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. Mr. Neal said the committee was considering legislation related to the issue.
“I take the authority to make this request very seriously, and I approach it with the utmost care and respect,” Mr. Neal said in a statement. “This request is about policy, not politics; my preparations were made on my own track and timeline, entirely independent of other activities in Congress and the administration.”
He added, “I trust that in this spirit, the I.R.S. will comply with federal law and furnish me with the requested documents in a timely manner.”
In addition to Mr. Trump’s personal returns for 2013 to 2018, Mr. Neal requested returns for Mr. Trump’s trust and seven other core Trump business entities that control scores of other Trump operations, including his golf club in Bedminster, N.J. He also asked the I.R.S. to share any information it had related to the entities, including whether they had been audited.
Liberal Democrats have complained for weeks that Mr. Neal, 70 and a roll-up-your-sleeves legislator, was dragging his feet on making the request. They have organized events in his district, taken out advertisements and produced legal briefs meant to make a case that he should act and act quickly.
Mr. Neal said throughout that he was chiefly concerned with crafting a request, alongside the House general counsel and the Ways and Means Committee staff, that could withstand legal challenge.
“I am certain we are within our legitimate legislative, legal and oversight rights,” he said on Wednesday.
In the Judiciary Committee, the chairman, Representative Jerrold Nadler of New York, said he would not immediately issue the subpoena for the Mueller report. But the party-line vote won by Democrats who control the committee ratchets up pressure on Attorney General William P. Barr as he decides how much of the nearly 400-page report to share with lawmakers.
“I will give him time to change his mind,” Mr. Nadler said in his opening statement. “But if we cannot reach an accommodation, then we will have no choice but to issue subpoenas for these materials.”
The committee also approved subpoenas for five former White House aides who Democrats said were relevant to an investigation into possible obstruction of justice, abuse of power and corruption within the Trump administration.
They included Donald F. McGahn II, a former White House counsel; Stephen K. Bannon, the president’s former chief strategist; Hope Hicks, a former White House communications director; Reince Priebus, the president’s first chief of staff; and Annie Donaldson, a deputy of Mr. McGahn.