House Democrats have given the Internal Revenue Service a new deadline to hand over President Donald Trump's tax returns days after Treasury Secretary Steven Mnuchin said his department would miss the original deadline of April 10.

In a letter to IRS Commissioner Charles Rettig sent Saturday by Ways and Means Committee Chairman Richard Neal, D-Mass., Democrats gave a second and final deadline of April 23 for six years of Trump's personal and business tax returns. The lawmakers could go to court to seek the returns if the IRS does not turn them over.

"To date, the IRS has failed to provide the requested return and return information despite an unambiguous legal obligation to do so ... Please know that, if you fail to comply, your failure will be interpreted as a denial of my request," Neal wrote in the letter.

The letter also states that there is "no valid basis to question the legitimacy of the Committee's legislative purpose," citing Supreme Court instructions that Congress' power to investigate is "broad."

"It is not the proper function of the IRS, Treasury, or Justice to question or second guess the motivations of the Committee or its reasonable determinations regarding its need for the requested tax returns and return information," Neal wrote.

The Ways and Means Committee first sent a formal request to the Treasury Department for Trump's tax returns on April 3, giving the department a deadline of April 10 to produce the documents.

"I today submitted to IRS Commissioner Rettig my request for six years of the president's personal tax returns as well as the returns for some of his business entities. We have completed the necessary groundwork for a request of this magnitude and I am certain we are within our legitimate legislative, legal, and oversight rights," Neal said in a statement.

But Treasury Secretary Steven Mnuchin pushed back on the request, saying his department would be unable to meet the deadline.

In a letter to Neal, Mnuchin said the Treasury was continuing to review Democrats' request in light of "serious issues" about whether the request is proper.

"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, saying he was consulting with the Justice Department about the legality and constitutionality of Neal's request.

Mnuchin said that "for the same reasons," he intended to supervise the department's review personally.

Trump's tax returns have been a contentious subject since he was a candidate for president. He has claimed that he cannot release his returns because he is under audit by the IRS.

However, in congressional testimony in February, Trump's former personal attorney Michael Cohen said he'd seen no proof that any audit was under way. And tax experts have said that even if he is under audit, there's nothing to stop Trump from releasing his returns.