Lawmakers heard from an expert Thursday who said there is “no wiggle room” for the Treasury secretary to deny a request for President Donald Trump’s tax returns from one of the tax committees.

A refusal to provide the tax returns would be “uncharted territory,” said George K. Yin, a professor at the University of Virginia School of Law and former chief of staff of the Joint Committee on Taxation. The panel spoke to several experts Thursday, probing them about the legality and history of requesting a president’s tax returns.

Yin spoke to the House Ways and Means Subcommittee on Oversight, saying that while the tax code bars revealing those returns without the filer’s permission, the tax returns can be shared with Congress.

But if the chairman of House Ways and Means wished to share that information with the House, he must have “a legitimate purpose” to do so because of the risk of disclosure in sharing the documents with the larger body, Yin said.

“During today’s hearing we will examine a topic of great interest to the American people,” said Rep. John Lewis, D-Ga., chairman of the House Ways and Means Subcommittee on Oversight, as he convened the hearing on issues relating to the disclosure of the president’s tax returns.

Ranking member Mike Kelly, R-Pa., said tax code rules that allow the committee to get the president’s tax returns were also written to ensure the privacy of all Americans’ returns.

For the Ways and Means Committee to use its right to demand the president’s tax returns would be “such an abuse of power” and it “would open up Pandora’s box.”

“Where does it end?” Kelly asked. “What about the tax returns of the speaker?”

But Rep. Tom Suozzi, D-N.Y., pointed to testimony from the experts that President Gerald Ford was the last president not to fully disclose his tax returns.

So, Suozzi said, there hasn’t been a need for the chairman of House Ways and Means to seek a president’s tax returns. Prior to Trump’s refusal to disclose his returns, it was more than 40 years since a president hadn’t voluntarily disclosed his tax returns.

The last candidate to capture more than 10 percent of the vote who didn’t disclose his tax returns was independent candidate Ross Perot in 1992, according to a Congressional Research Service report. The last major party candidate to not disclose tax returns was Democrat George McGovern in 1972, although President Richard Nixon disclosed his returns.

In 1974, both major party candidates disclosed tax returns for the first time, according to CRS, though President Gerald Ford released only “partial” tax returns. President Jimmy Carter disclosed “full” returns.

Since then, every president from Carter to Barack Obama _ Ronald Reagan, George H.W. Bush, Bill Clinton and George W. Bush _ all disclosed their returns.

The last vice presidential candidate of a major party to not publicly provide tax returns was Ford’s 1976 running mate, former Sen. Robert Dole. During the 2016 campaign, Vice President Mike Pence released 10 years of his tax returns covering 2006-2015.

Rep Jackie Walorski, R-Ind., said Thursday that “the level of disclosure varies widely” among the presidents from Ford to Obama who did release their returns. And tax returns of a business owned by a president have not been disclosed, she pointed out.

Joseph J. Thorndike, director of the Tax History Project, agreed that presidential candidates disclosing their tax returns is “tradition, not a law.”

He noted, though, that former Republican presidential candidate Mitt Romney in 2012 disclosed some tax return information from trusts in which he had ownership.

“The American people deserve to know whether their executive stands to personally benefit” or be influenced because of his business dealings, said Rep. Linda T. Sanchez, D-Calif.

Rep. Darin LaHood, R-Ill., quizzed the panel of experts whether a chairman of the Ways and Means Committee has ever sought a president’s tax return while there was an ongoing Justice Department independent counsel investigation. None could think of an example. “It’s confusing to me,” LaHood said. “It’s never been done. This seems to me like a waste of time and resources.”