After the committee originally requested the tax returns from IRS Commissioner Charles Rettig in April, Mnuchin set his own deadline for May 6, taking the extra time to review with the Justice Department whether he was even legally required to hand over the returns. This move greatly angered Democrats on the House Ways and Means Committee, who have repeatedly maintained they have full authority to demand tax returns from Trump and Vice President Mike Pence.

The committee is conducting the review for the purpose of examining the IRS process for auditing presidential tax returns. This conflicts with Trump's claim that he cannot release his returns until he is done being audited.

Mnuchin also said that the Justice Department would publish a full legal review of the decision and said the Treasury could "provide information concerning the Committee's stated interest in how the IRS conducts mandatory examinations of Presidents."

Democrats now have several options before them, including a subpoena for the returns. So far, each request has been voluntary. If Trump administration officials decline to comply with the subpoena, a lawsuit could follow.

In a statement following Mnuchin's letter, House Ways and Means Committee Chairman Richard Neal offered a cryptic response to the decision.

"Today, Secretary Mnuchin notified me that the IRS will not provide the documents I requested under Section 6103 of the Internal Revenue Code. I will consult with counsel and determine the appropriate response," Neal said in a statement.

Read more: Tax returns show many 2020 Democrats have one financial habit in common

Mick Mulvaney, Trump's acting chief of staff, has said Democrats would "never" obtain Trump's taxes, echoing many of the president's allies who insist that because Trump broke precedent by not voluntarily releasing his returns as a presidential candidate in 2016 and still won the election, the issue is settled in the minds of the public.

Republicans on the House Ways and Means Committee have said that it is an abuse of power to expose people's tax returns, including the president's. This is despite the fact that Republicans on the same committee revealed the private tax information of a handful of US citizens during a 2014 probe when they were in the majority.

And Trump himself, who has made a concerted effort to deny Democrats various probes into his administration and personal businesses, has repeatedly said he would not reveal his tax returns.