Trump has broken with decades of presidential precedent by not voluntarily releasing his returns to the public.
Mnuchin, who has consulted with the White House and Department of Justice about Trump's tax returns, said earlier this week that Neal's request raised concerns about the scope of the committee's authority, privacy protections for USA taxpayers and the legislative objective of lawmakers in seeking the documents.
On Wednesday, Mnuchin said the Treasury was handling the request and that a deadline set for that date would be missed, citing possible constitutional issues.
The president's former personal lawyer, Michael Cohen, told a House panel in February that he does not believe Trump's taxes are under audit.
"Democrats are amped up, we are fired up to highlight issues that matter to folks in Wisconsin", Bowen said.
Neal said his agency was consulting with the Justice Department to determine whether the request for Mr. Trump's tax returns is legal and constitutional.
"I am aware that concerns have been raised regarding my request and the authority of the Committee", Neal wrote to IRS Commissioner Charles Rettig.
Mnuchin told lawmakers that his department will "follow the law", but he hasn't shared the department's interpretation of the statute.More news: How Israel’s Netanyahu Uses Fear and Loathing to Win Elections
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Highlighting what he called the "unprecedented nature of this request", Treasury Secretary Steve Mnuchin wrote to Neal that he - not the IRS commissioner - would manage the Treasury's handling of the request and that the Justice Department was being consulted "to ensure that our response is fully consistent with the law and Constitution". The IRS says there's no rule against subjects of an audit from publicly releasing their tax filings.
"Together, we are going to make sure that does not happen again", Sanders said to cheers.
Trump earlier falsely claimed that "there's no law whatsoever", requiring him to hand over his taxes to Congress, although the law clearly states the opposite.
Trump appears prepared to fight this to the Supreme Court. If Rettig fails to do so, Neal said he will interpret as denying the request, which could pave the way for a court battle.
Neal has adopted a methodical approach to seeking Trump's returns.
On Saturday, Neal said the administration has no right "to question or second guess" his motivations.
"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", wrote Mnuchin in his explanation for why the April 10 deadline could not be met.