Of the dozens of potential investigations House Democrats may pursue in 2019, few have received as much coverage as the possibility of examining President Donald Trump’s tax returns.

Mr Trump repeatedly promised – then refused – to release his returns in 2016. Democrats on the House Ways and Means Committee now plan to formally request them from the treasury secretary during the 116th Congress.

“We’re keeping close-to-the-vest when we’re going to do this and how we’re going to do this,” Representative Bill Pascrell Jr, said in an interview with The Washington Post last week, “but leadership agrees that this should be one of our priorities.”

Mr Pascrell has led 17 separate Democratic attempts over the past two years to obtain Mr Trump’s tax returns via a little-known 1924 provision in the Internal Revenue Code that was added during America's Teapot Dome bribery scandal.

The relevant part (Section 6103 F1) says the treasury secretary “shall furnish” the tax return of any individual upon written request from the chairman of the Ways and Means Committee, the Senate Finance Committee or the Joint Committee on Taxation.

George Yin, a University of Virginia law professor and the former executive director on the Joint Committee on Taxation, told The Washington Post that while the 6103 authority does not require a congressional subpoena, it could be considered analogous to a subpoena and therefore would likely require a legitimate legislative purpose if Mr Trump or the Treasury Department challenged it in the courts.

According to Mr Pascrell, Democrats plan to cite Mr Trump’s “potential conflicts of interest” domestically and abroad, including whether Mr Trump is in violation of the emoluments clause of the Constitution, as the legislative purpose. Mr Pascrell says potential conflicts could put the president in a “compromising position.”

“I’d like to go back 15 to 20 years [in Trump’s returns], and I don’t say that haphazardly, because we want to know how involved he was in these [business dealings],” Mr Pascrell said, adding he did not think there were limits on how much return information the committee could request.

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Even if the Ways and Means Committee gets Mr Trump’s returns, it may not provide a full picture of Mr Trump’s foreign business dealings. And the public would not see Mr Trump’s returns unless the full House voted to release them after being submitted by the committee, something Mr Pascrell says Democrats will discuss.

The provision has been used only a handful of times over the past century, including the investigation into whether the IRS targeted tax-exempt political groups in 2014 and the congressional investigation into President Richard Nixon’s returns in 1974 that resulted in Mr Nixon paying over $400,000 in back taxes to the IRS.

In November, Mr Trump warned of a “war-like posture” if Democrats requested his tax returns. Mr Yin, who has consulted with Mr Pascrell on obtaining Mr Trump’s taxes, is unaware of any court challenges to the 6103 provision, but Democrats acknowledge Mr Trump could try to block the release of his returns, which could lead to a court battle.

“This is not a way to embarrass the president. He does a good job at that himself,” Mr Pascrell said. “The chairman has the right to ask for those returns and the president may feel that there’s no law which forces him to do that and then we’ll see. We’ll go to the courts.”