Spokesmen for the White House and the Treasury Department did not immediately respond to CNBC's requests for comment on the subpoenas.

"While I do not take this step lightly, I believe this action gives us the best opportunity to succeed and obtain the requested material," Neal wrote. "I sincerely hope that the Treasury Department will furnish the requested material in the next week so the committee can quickly begin its work."

But Neal, in the letter announcing the subpoenas Friday afternoon, called his request for the returns "reasonable." The subpoenas give Rettig and Mnuchin until May 17 to return Trump's tax information.

The subpoenas came four days after Mnuchin said that he would not allow Trump's personal and business tax returns to be released to Congress because he had determined that the request "lacks a legitimate legislative purpose."

House Ways and Means Committee Chairman Richard Neal, D-Mass., said Friday that he has issued subpoenas to Treasury Secretary Steven Mnuchin and IRS Commissioner Charles Rettig demanding six years of President Donald Trump 's federal tax returns.

Trump has said his administration plans to fight "all the subpoenas" issued by House Democrats. House committees led by Democrats have issued subpoenas on a range of issues, demanding financial records and counterintelligence information, among other requests.

In late April, Trump and his business filed suit against House Oversight Committee Chairman Elijah Cummings, D-Md., to block a subpoena seeking information about Trump's finances.

Trump has claimed, as far back as the 2016 presidential campaign, that he cannot turn over his returns because they are being audited. But there is no law precluding him from releasing them during an audit.

After winning the presidency, his senior advisor, Kellyanne Conway, said Trump was "not going to release his tax returns" because "people didn't care."

Ways and Means Committee member Bill Pascrell, D-N.J., said the subpoenas were necessary "only because Trump, Mnuchin, and Rettig have signaled their disdain for our laws over and over again."

Neal has said his committee, along with two others in Congress, is entitled to look at any taxpayer's returns under a section of the IRS Code that says the executive branch "shall furnish" Congress with a taxpayer's information upon request.

The House also is moving toward issuing a subpoena seeking to compel Attorney General William Barr to turn over the full, unredacted report by special counsel Robert Mueller and the underlying evidence obtained as part of his investigation into Russian interference in the 2016 presidential election and related issues.

That committee, led by Chairman Jerrold Nadler, D-N.Y., voted to hold Barr in contempt Wednesday for the Justice Department's refusal to comply with the Democrats' subpoena.

On Wednesday, the New York State Senate passed a bill that would allow the state's tax agency to release any state tax return requested by the House Ways and Means Committee, as well as the U.S. Senate Finance Committee or the Joint Committee on Taxation.

The move could pave the way for New York to send Congress the president's state tax returns, which are likely to contain much of the same information that appears on his federal tax returns.

Read the full press release from Neal below:

WASHINGTON, DC – Today, Ways & Means Committee Chairman Richard E. Neal (D-MA) released the following statement after issuing subpoenas to Department of Treasury Secretary Steven Mnuchin and Internal Revenue Service (IRS) Commissioner Charles Rettig for six years of the President's personal and business tax returns: "Last month, the Ways and Means Committee began an investigation into the mandatory audit program at the IRS in an effort to assess the extent to which the IRS audits and enforces the federal tax laws against a sitting President and to determine if those audits need to be codified into federal law. As part of that inquiry, on April 3, I requested six years of the President's personal and business tax returns, pursuant to my authority under section 6103(f) of the IRS Code. I believed then, as I do now, that reviewing the requested documents is a necessary piece of the committee's work. "Unfortunately, the Treasury Department and the IRS have denied my reasonable request. Despite the agency's denial, we on the Ways and Means Committee are still moving forward with our inquiry. After reviewing the options available to me, and upon the advice of counsel, I issued subpoenas today to the Secretary of the Treasury and the Commissioner of the IRS for six years of personal and business returns. While I do not take this step lightly, I believe this action gives us the best opportunity to succeed and obtain the requested material. I sincerely hope that the Treasury Department will furnish the requested material in the next week so the committee can quickly begin its work."

Read the subpoenas to Mnuchin and Rettig below: