House Democrats began Thursday the careful process of creating an official record for why they should be allowed to see President Donald Trump’s tax returns.

Democrats first move toward obtaining the Trump tax returns came at a Ways and Means subcommittee hearing ostensibly devoted to a portion of a new legislative package that would force presidential and vice presidential candidates to disclose 10 years of their tax returns.

But, expecting a vigorous fight with White House that will likely wind up in federal court, Democrats on the House Ways and Means Committee spent the afternoon piecing together a legal and political argument for their right to obtain the highly sought tax returns.

Using hearing testimony from expert witnesses, Democrats on the committee argued that tax law gives Ways and Means the right to make the request for the tax returns, and that Congress would be exercising a legitimate use of its oversight function by seeking them from Treasury Secretary Steve Mnuchin.

“In other words, we will ask the question: Does the public have the need to know that a person seeking or holding the highest office in the country obeys the tax laws?” said subcommittee chair John Lewis (D-GA) at the hearing.

Experts and former congressional investigators told TPM this week that the hearing could be used in a later, anticipated court case over the request to establish a “legitimate purpose” on the part of Congress to ask for the returns.

“I don’t see any wiggle room in the statute to refuse the request,” said UVA law professor and former Democratic joint committee on taxation staffer George Yin, a committee witness. “Should the secretary refuse, we would be in uncharted territory.”

He added to the members of Congress that if the case goes to court, previous law “indicate[s] that Congress must act with a legitimate legislative purpose, meaning a purpose in line with its congressional responsibilities.”

Joseph Thorndike, director of the Tax History Project, argued at the hearing that the last time the committee gained access to a president’s tax returns – involving Richard Nixon – it revealed possible malfeasance on the part of the IRS.

“Nixon’s returns had been audited twice,” Thorndike said, with the IRS giving Nixon a complimentary note after the first audit.

The second audit, conducted with the committee, found unpaid back taxes of nearly $480,000.

“That episode underscored a key question: can we rely on the IRS to fairly and impartially enforce the tax laws” when it comes to a sitting President?

Republicans tried to steer the debate towards whether the raw returns themselves will be disclosed to the public – an unlikely eventuality that is separate from what is likely to be the committee’s request.

At one point, Rep. Suzan Delbene (D-WA), asked a witness – former Republican joint tax committee general counsel Ken Keis – whether he would have “advised Republicans to release taxpayer information in 2014.” Delbene was referring to an episode in the 2014 IRS targeting investigation, where the committee, under the leadership of Paul Ryan, requested and disclosed the returns of 51 taxpayers.

“No, I would not have advised them to do that,” Keis said.