The White House is committed to letting the negotiations play out, with some saying they are "cautiously optimistic" about getting a deal they could live with, said a senior administration official who spoke on condition of anonymity.
Thursday's hearing is created to build a case for Neal's authority to request Trump's returns, said Andy Grewal, a tax law professor at the University of Iowa.
A fractious afternoon hearing of a House Ways and Means oversight subcommittee was intended to begin building a case that Trump's withholding of his returns was not only flouting modern political norms but also potentially hiding violations of federal tax laws and compromising the interests of the United States.
That goal has been high on their list of priorities since they won control of the House in November's midterm elections, but asking for Trump's returns is likely to set off a huge legal battle with his administration.
"Democrats pledged to obtain President Trump's tax returns on the first day of the new Congress - and we intend to hold them accountable for that promise", said Tax March spokesman Ryan Thomas.
The House Judiciary Committee voted along party lines Thursday morning to give its chairman the authority to subpoena testimony from Whitaker should he fail to show up or answer questions during Friday's hearing. But Kay Granger of Texas, the top Republican on House Appropriations, said negotiations may go past Friday but be completed in time to avert a shutdown.
Rep. Steven M. Palazzo, R-Miss., a conference committee member, said he expects a deal will be reached "before the end of the weekend" and that aid for disaster victims would be included. "They talked about this when they were running, so we do need to make sure that the American people know we are looking out for what we've had from every other president".
Shelby declined to detail Trump's requirements, and he did not say whether the president had agreed to accept less than the $5.7 billion he's been demanding to build new steel walls along the U.S. -Mexico border. It could take years to resolve, possibly stretching beyond the 2020 presidential election.
No. 2 House GOP leader Steve Scalise of Louisiana told reporters that "from the things that I'm hearing, this agreement could get a majority of both Republicans and Democrats in the House". "There is no end in sight for those whose tax information may be in jeopardy".
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". "It would set a very unsafe precedent".
In January, Neal told CNN that he is "judiciously" pursuing Trump's tax returns. One provision of the law lets the Defence Department provide support for counter-drug activities. A Treasury spokesperson said Mnuchin would review any request for the President's tax returns with his general counsel to determine if they are required under law.
Trump has cited IRS audits of his personal tax returns as a reason not to release them to the public. "In terms of the tax issue, it's not a question of just sending a letter".
But it soon became the first real skirmish between Trump's defenders in Congress and newly empowered Democrats since the House changed hands last month.
The oversight hearing, led by subcommittee chair Rep. John Lewis, D-Ga., will also examine current tax law regarding presidents' tax returns.
Lawmakers could then eventually make the records public, if they so choose. Trump's lawyer Rudy Giuliani has made it clear that the President intends to fight any request in court.
"The safest place for Democrats to be on anything Trump-related is in the fact zone", said Phil Singer, a former communications adviser to Democrats.
"I don't see any wiggle room in the statute for the secretary to refuse a request", Yin said.
"Weaponizing the tax code for political purposes sets a unsafe precedent", Brady, R-Texas, told reporters on Wednesday.