U.S. President Donald Trump listens during a White House Opportunity and Revitalization Council meeting at the White House in Washington, D.C., U.S., on April 4, 2019.

"Even if Ways and Means had a legitimate committee purpose for requesting the President's tax returns and return information, that purpose is not driving Chairman [Richard] Neal's request. His request is a transparent effort by one political party to harass an official from the other party because they dislike his politics and speech," Consovoy argued in the letter obtained by NBC News.

In a letter Friday to Treasury Department General Counsel Brent McIntosh, lawyer William Consovoy argued that the House Ways and Means Committee has no "legitimate committee purpose" for seeking the president's returns. The attorney from the law firm Consovoy McCarthy Park also wrote that the Treasury would set "set a dangerous precedent" by releasing the returns.

The letter escalates the legal dispute between the White House and House Democrats, who formally requested the president's tax returns on Wednesday evening. Trump, who came into office in 2017 still owning sprawling global business holdings, did not release his returns during the 2016 election, breaking with decades of precedent. Democrats have said politics did not inspire their request.

Neal, D-Mass., chairman of the House Ways and Means Committee, asked the IRS, which is part of the Treasury, for six years of the president's personal tax returns and some of those for his businesses. It was the first time Congress asked for a sitting president's tax information in 45 years.

"I take the authority to make this request very seriously, and I approach it with the utmost care and respect," Neal said in a statement Wednesday. "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."

Trump has repeatedly claimed he cannot release his returns because he is under audit. Asked Wednesday about sending his tax returns to Congress, he said that "until such time as I as I am not under audit, I will not be doing that."

Some legal experts have disputed the argument.

On Wednesday, Neal in part said his committee "has a duty to examine whether Congressional action may be needed to require such audits, and to oversee that they are conducted properly."

In his letter, Consovoy argued "it would be a gross abuse of power for the majority party to use tax returns as a weapon to attack, harass, and intimidate their political opponents."

"Once this Pandora's box is opened, the ensuing tit-for-tat will do lasting damage to our nation," he writes. "Can the Chairman request the returns of his primary opponents? His general-election opponents? Judges who are hearing his case?"

Spokespeople for the Treasury Department and Neal did not immediately respond to requests to comment on the letter.

Democrats have wanted to see the president's tax returns in order to determine whether his personal holdings pose any conflicts with his policymaking. Polls have found broad public support for Trump releasing his tax returns.

Sixty-seven percent of respondents to a Quinnipiac University poll in February said they think Trump should publicly release his tax returns, versus 24 percent who said he should not. Fifty-two percent of respondents said they think he has not released his tax returns "because he has something to hide," while 19 percent responded that he had not done so because he was being audited.

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