Democratic leaders are under growing pressure to immediately request President Donald Trump's tax returns as the party flexes its newfound oversight authority.

Democrats plan to pursue a broad range of investigations into possible conflicts of interest and ethics violations within the Trump administration. Just this week, key committees took action to expand cooperation with special counsel Robert Mueller's probe and grill top administration officials.

Securing the president's tax returns is a crucial part of that effort, a promise invoked since Trump broke with decades of precedent and refused to release them during the 2016 election. But Democrats have yet to request the documents from the administration.

On Thursday, a key branch of the powerful House House Ways and Means Committee is slated to hold a hearing on presidential tax returns. Democrats will call for presidential and vice presidential candidates to disclose 10 years of tax returns as part of a comprehensive ethics reform package known as HR1.

But progressives are urging lawmakers — in particular, Chairman Richard Neal, D-Mass. — to take a more strident stance.

"While we strongly support the inclusion of this provision in HR1 to ensure that tax return disclosure is forever a requirement for presidents, we also want to make sure that the committee focuses in on Trump explicitly and uses their granted authority to get his now," said Lisa Gilbert, vice president of legislative affairs at Public Citizen, a watchdog group.

Neal is also facing critical TV ads in his home district. Need to Impeach, the activist organization funded by billionaire Tom Steyer, is running 30-second spots calling on Neal to go after Trump's returns and begin impeachment proceedings. The group said it intends to run similar ads targeting House Judiciary Chairman Jerry Nadler, D-N.Y., next week, followed by campaigns against rank-and-file members of key committees.

Kevin Mack, the group's lead strategist, said it is committing $40 million to fund its current efforts. It is also launching a direct mail campaign and getting on the ground to knock on doors.

"Voters went to the polls in record numbers because they want to hold Donald Trump accountable," he said. "I think everyone's a little surprised that Democrats are saying they're going to wait for some unforeseen time to take action."

Over the past few weeks, Neal has publicly stated that he does intend to request Trump's tax returns but has not laid out a timeline for doing so.

"We feel that this is exceptionally important to get right," Neal spokesman Dan Rubin said. "We'd rather do it right than do it on day one."

Under the law, the chairman of the Ways and Means Committee has the authority to request a confidential review of the tax returns of any individual. The committee would have to vote to allow members access to the information — and even then, it can only be viewed in closed executive session unless the taxpayer agrees to make the return public. Violating that confidentiality is punishable by a fine of up to $5,000 and five years in prison.

"Weaponizing the tax code for political purposes sets a dangerous precedent," Rep. Kevin Brady of Texas, the ranking Republican on the committee, told reporters Wednesday. "If Democrats or any party can abuse their power to rummage through the tax returns of the president, what will stop them from abusing that power in the future to frankly target any individual American that they see as a political enemy?"

The law does not provide a deadline for the Treasury Department to hand over the president's returns once a request is made. In addition, any request is almost certain to be met with a court challenge. A Treasury spokeswoman said Secretary Steven Mnuchin will review any request with the department's general counsel for legality.

Democratic aides say the purpose of Thursday's hearing is to begin building the public case for requesting the president's tax returns, which could serve as a bulwark against potential litigation. Democrats are also still mapping out their plan of action for holding the Trump administration accountable. With multiple committees wielding oversight authority, leadership is attempting to coordinate their work.

"We don't want to be scattershot," House Majority Leader Steny Hoyer told reporters Wednesday. "We want to do oversight, which is substantive and appropriate."