Secretary of Treasury Steve Mnuchin testifies at the U.S. Capitol on May 15, 2019 in Washington, DC.

Treasury Secretary Steven Mnuchin and IRS Commissioner Charles Rettig are expected to defy congressional subpoenas requesting years of President Donald Trump's federal tax returns by Friday afternoon.

By ignoring the 5 p.m. ET deadline, the administration officials would spark the latest point of contention between Democrats, who have lodged dozens of subpoenas for information from the White House and Trump-related figures, and the president, who has vowed to fight "all the subpoenas."

If the deadline isn't met, the battle over the president's tax information could be headed to the courts.

House Ways and Means Committee Chairman Richard Neal, D-Mass., made a formal request for six years of Trump's personal and business tax returns last month. But Mnuchin refused to release the president's returns to Congress.

Last Friday, Neal issued subpoenas to Mnuchin and Rettig for the 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," Neal said in a statement.

Hours before the deadline, Neal told reporters on Capitol Hill that he anticipated Mnuchin and Rettig would blow off the subpoenas. "The result will be that we will likely proceed to court as quickly as next week," Neal said.

Asked if he foresaw his committee moving to hold Mnuchin in contempt for defying the subpoenas, Neal said "I don't see that right now as an option."

The debate has focused on a specific section of the federal tax code, which states that the Treasury secretary "shall furnish" an individual's tax returns when a written request from the tax committee is made. Mnuchin says that the request "lacks a legitimate legislative purpose" and that it could therefore not "lawfully" be fulfilled.

At a Senate Appropriations subcommittee hearing Wednesday, Mnuchin strongly suggested that he and the IRS chief will not cooperate with those subpoenas. Lawmakers can "guess which way we're leaning" on how he and Rettig will respond to the Democrats' requests, Mnuchin testified.

The Treasury Department did not immediately respond to CNBC's request for comment on whether or not the deadline will be met.

It would be better to turn to the courts to try and hash out the "difference in interpretation" between Democrats and the Trump administration, Mnuchin said at that hearing. "This is why there are three branches of government, so if there is a difference of opinion this will go to the third branch of government to be resolved."

Mnuchin and Republicans on the Ways and Means Committee have both claimed that Democratic lawmakers are attempting to "weaponize" the IRS in order to damage Trump, rather than use their powers to further a legislative interest.

"It has become obvious that your supposed legislative purpose is just a pretext, and your request is merely a means to access and make public the tax returns of a single individual for purely political purposes," said Rep. Kevin Brady, R-Texas, the Ways and Means Committee's ranking member, in a letter urging Neal to let go of his pursuit for the tax returns.

Neal, however, argued that the request was "reasonable" and that reviewing the president's tax returns constitutes "a necessary piece of the committee's work" as an oversight body. Trump's returns would be used as part of 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," Neal said.

Trump refused to make his tax returns publicly available as a presidential candidate, breaking with the precedent set by most other candidates in recent decades. Trump claimed during the campaign and after the election that he could not release his returns until the completion of an audit — even though there is no legal barrier to him sharing that information while being audited.