"Any of the [tax] committees can act unilaterally, and so if the Democrats simply took the House, not the Senate, the House [Ways and Means Committee] under the authority of the House could proceed on its own. Similarly, the Senate could proceed on its own if the opposite situation were to occur."

"The law authorizes the chair of any one of three tax committees in Congress to request the tax-return information of any person or any business, and directs the secretary of the Treasury to furnish the information that's requested," Yin says. "Upon receipt, the committee must examine the information in a confidential manner. But it can appoint agents such as staff or outside experts."

Here & Now's Jeremy Hobson talks with George Yin , professor of law and taxation at the University of Virginia School of Law and former chief of staff of the Congressional Joint Committee on Taxation, about the law surrounding the issue: 26 U.S. Code § 6103 .

But if Democrats win control of the House of Representatives in November and assume the chair of the Ways and Means Committee, they may be able to get the Treasury Department to release Trump's tax returns.

"The long history — going back 40 years in this country — has been that every president has voluntarily turned over his tax returns. There was a good public purpose, if you will, for that. And so the committee could be carrying that out."

On if the public would be able to see the tax returns if the House Ways and Means Committee obtained them

"There is a further provision in the law that allows the committee, after completing its investigation, to submit any of the information that it's obtained to the full House or the full Senate, and presumably it would be by some kind of committee vote. The committee would resolve to do this. Now again, the law doesn't specify, but I think sound practice would say that there should be some reason why the committee feels it needs to share this more broadly, beyond simply the committee and its staff.

"There are any number of reasons. It really depends a little bit on what the initial investigation to start out with is. This is a tax committee, and so it certainly could be a legitimate reason to examine, because the committee has oversight over the tax function. The long history — going back 40 years in this country — has been that every president has voluntarily turned over his tax returns. There was a good public purpose, if you will, for that. And so the committee could be carrying that out.

"Beyond that, obviously there have been stories. The New York Times had a story about some of the ... prior tax filing by Mr. Trump's father. There have been obviously issues about potential conflicts of interest, things of that nature."

On the history of this law

"It is almost a 100-year-old law, and the circumstances when it came about were actually fairly close to our current circumstances. The law prior to this law allowed only the president to make a decision of whose tax information could be disclosed. At that time when that law was in effect, it frustrated Congress in the early 1920s.

"There were a variety of reasons. Two principal ones were they were carrying out a number of investigations of the executive branch, including the scandal that became known as the Teapot Dome scandal, involving some high-level executive branch officials. The committees that were investigating wanted to examine their tax returns, and because of the law, they had to actually go to the president and request permission from the president to get the tax material — even though they were investigating you know executives under the president. And so obviously, Congress realized, 'Gee, we're not really a co-equal branch of government if we have to go through this kind of procedure.'

"There was a similar concern about Treasury Secretary Andrew Mellon, who was an extremely wealthy and successful businessperson prior to going into government, and like the president, did not surrender or place into blind trust his business interests, but simply retained them while he was in office. And that obviously then raised all sorts of questions in Congress's mind. The Coolidge administration at that time was pushing through various tax legislation, just like the existing administration pushed through at the end of last year, and there were issues of, 'Well, how would this legislation affect you personally? How would it affect your businesses?' So there was all of that going on, and Congress ultimately decided to be a co-equal branch of government. It needed to have the authority to get this information directly, just as the executive branch already had."