Democrats say they got a tip from a federal employee that somebody may have tried to interfere with an IRS audit of President Donald Trump.

The revelation came to light in a court filing in Democrats’ lawsuit seeking the president’s tax returns.

In late July, the House Committee on Ways and Means “received an unsolicited communication from a federal employee setting forth credible allegations of ‘evidence of possible misconduct’― specifically, potential ‘inappropriate efforts to influence’ the mandatory audit program,” the Tuesday court filing says.

The IRS automatically audits the president and vice president every year. Democrats have said they need copies of Trump’s tax returns in part to make sure the audit is legit.

Democrats sued in July after the Trump administration refused to comply with a federal tax disclosure law that has entitled tax committee chairs to private tax information since 1924.

Their filing in federal court this week argued that District Judge Trevor McFadden should order the Treasury Department to hand over Trump’s tax returns without a trial, since the disclosure law gives no leeway to refuse. And they say the whistleblower’s material bolsters their case.

The Trump administration responded that the request for Trump’s taxes raises weighty issues about whether Congress is overstepping its constitutional powers, and that the court should take its time. Past administrations have benefited from federal court cases taking longer than election cycles.

In an Aug. 8 letter to Treasury Secretary Steve Mnuchin, Ways and Means Chairman Richard Neal (D-Mass.) called the tipster’s claim a “grave charge” that cast doubt on Mnuchin’s argument that there’s no reason to worry IRS agents could be subject to political influence. He asked for a “a rolling production of documents and communications of specified Treasury and IRS employees.”

Mnuchin refused to cooperate, saying he forwarded Neal’s letter to the Treasury Inspector General for Tax Administration, an internal agency watchdog.

A Treasury spokesman declined to comment.

The tax return case is one of several legal battles between Congress and the administration that could have major consequences for the U.S. system of government. In the past, courts have given lawmakers wide latitude for investigations, but Trump has stonewalled to an unprecedented degree.

Read the filing below: