President Trump responded with a dismissive taunt on Wednesday after a House committee chairman formally requested the IRS provide several years of his personal and business tax returns, in a move that prompted congressional Republicans to warn that Democrats had "weaponized" tax law.

Told by a reporter at the White House that Democrats wanted six years of his tax returns, Trump replied: "Is that all? Usually it's 10. So I guess they're giving up. We're under audit, despite what people said, and we're working that out -- I'm always under audit, it seems, but I've been under audit for many years, because the numbers are big, and I guess when you have a name, you're audited. But until such time as I'm not under audit, I would not be inclined to do that."

The request Wednesday by Massachusetts Rep. Richard Neal, who heads the tax-writing House Ways and Means Committee, is the first such demand for a sitting president's tax information in 45 years. The move sets up a virtually certain legal showdown with the White House.

Neal made the request in a letter to IRS Commissioner Charles Rettig, asking for Trump's personal and business returns for 2013 through 2018. Neal told Rettig that Democrats have a duty "to ensure that the Internal Revenue Service is enforcing the laws in a fair and impartial manner."

“It is critical to ensure the accountability of our government and elected officials," Neal said in a statement. "To maintain trust in our democracy, the American people must be assured that their government is operating properly, as laws intend."

The president's congressional allies registered immediate and fierce disapproval. The top Republican on the House Ways and Means Committee, Kevin Brady, R-Texas, wrote to Treasury Secretary Steven Mnuchin to decry what he called Democrats' "abuse" of their authority.

“Weaponizing our nation’s tax code by targeting political foes sets a dangerous precedent and weakens Americans' privacy rights, As you know, by law all Americans have a fundamental right to the privacy of the personal information found in their tax returns," Brady said in the letter. “This particular request is an abuse of the tax-writing committees’ statutory authority, and violates the intent and safeguards of Section 6103 of the Internal Revenue Code as Congress intended.”

That provision of tax law generally prohibits the disclosure of personal tax information.

Brady added that while "transparency in our government is enormously important," the "privacy and freedom" of all taxpayers is paramount -- and that Congress should pass new disclosure laws if it sees a problem. Violating the privacy rights of one taxpayer, Brady asserted, "begins the process of eroding and threatening the privacy rights of all taxpayers."

A spokesperson for Senate Finance Committee Chairman Chuck Grassley, R-Iowa, told Fox News that the "ability of the chairman to request such information is intended to inform the legislative process, which is how it’s been used in the past, not to engage in a politically-motivated fishing expedition."

Congress "passed section 6103 of the tax code to prevent that kind of abuse of power and to protect every taxpayer’s privacy," the spokesperson continued. "Those seeking an individual’s personal tax returns to exact political damage would be opening the door to future abuses of power and would poison the public trust in the ability of the IRS to keep personal information private. That’s an outcome every taxpayer and their elected representatives should want to avoid."

Neal specifically demanded the federal income tax returns from eight entities, including Trump National Golf Club-Bedminster, as well as statements specifying whether the returns were ever under audit. Neal also demanded all administrative files, including affidavits, related to each return.

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Senate Finance Committee Ranking Member Ron Wyden, D-Ore., followed up with a statement backing up his counterpart in the House.

“The law is crystal clear—the Treasury Department must provide tax returns to the Ways & Means and Finance Committees when the chairman requests them. I expect the Treasury Department to comply in a timely manner,” Wyden said. “Chairman Grassley should make the same request so Senate Finance Committee members are also able to access them.”

Fox News' Mike Emanuel, Chad Pergram and The Associated Press contributed to this report.