“Please know that, if you fail to comply, your failure will be interpreted as a denial of my request,” Neal concluded the letter.

“I am aware that concerns have been raised regarding my request and the authority of the Committee. Those concerns lack merit,” House Ways and Means Committee chairman Richard Neal (D-Mass.) wrote in a letter to Internal Revenue Service Commissioner Charles Rettig, three days after the White House missed his initial deadline .

The White House will now have until April 23 to comply or face a potentially lengthy legal challenge.

WASHINGTON ― Democrats gave President Donald Trump ’s administration a new deadline to hand over copies of the president’s tax returns on Saturday after the administration refused to provide the documents last week .

This morning, chairman @RepRichardNeal sent the following letter to IRS commissioner Rettig after the IRS missed the April 10th deadline to furnish six years of the president’s personal & business #taxreturns : pic.twitter.com/aHSNvEJwk3

Neal is seeking six years of Trump’s personal and business tax returns under a tax code provision that gives the chairs of congressional tax committees access to private tax information. The law gives no leeway for the administration to refuse, but Treasury Secretary Steve Mnuchin said last week that he thought Neal’s request was unconstitutional.

“The legal implications of this request could affect protections for all Americans against politically-motivated disclosures of personal tax information, regardless of which party is in power,” Mnuchin wrote.

The back-and-forth letters are setting the stage for an unprecedented legal fight ― experts say no administration has ever refused to comply with the law before. Democrats should be in a strong position given the broad authority Congress has to investigate the executive branch, but Republicans can still run out the clock.

Republicans have been trashing Neal’s request as an abuse of power and invasion of privacy, even though they themselves have sought private tax information under the law several times. In February, the Senate Finance Committee sought private tax information related to tax-exempt hospitals, and in 2014 House Republicans obtained and disclosed private tax info as part of an investigation into the IRS.

For his part, Trump has for years claimed he is under audit and that his lawyers have advised him not to releases his tax returns.

On Tuesday, however, Rettig undermined the president by confirming to the House Appropriations Subcommittee that there is “no rule that would prohibit the release of a tax return because it’s under audit.”

Polling has consistently found that most Americans would like to see the tax returns.

According to the results of a HuffPost/YouGov poll released this week, Americans say, 56% to 27%, that Trump should release his tax returns to the public.