A federal employee told the House Ways and Means Committee of possible efforts to improperly influence the process used by the IRS to audit presidential tax returns, according to documents filed Tuesday in federal court in Washington.

The allegation was included in a motion from the committee asking a federal judge to rule, without a trial, that the Treasury Department is legally obligated to disclose President Donald Trump's tax returns. Lawyers for the committee said the case is so straightforward that the judge can rule based solely on written submissions from both sides.

The Ways and Means Committee, led by Rep. Richard Neal, D-Mass., sued the Treasury Department in July after Secretary Steven Mnuchin refused the committee's request for Trump's returns. Neal said the committee is investigating whether the IRS is properly conducting required audits of a president's and vice president's returns.

That concern was heightened, Neal said, when the committee "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.’"

Politico first reported the Democrats' disclosure about the whistleblower's allegations.

IRS internal regulations require an automatic audit of the tax returns of a president and vice president. The committee's lawsuit said it sought Trump's returns to discover whether the IRS was appropriately carrying out that function.

Mnuchin said in response that the whistleblower's allegations have been referred to the Treasury Department's inspector general for tax administration.

The Trump administration has urged the judge to reject the committee's efforts to obtain the returns, which the government said were animated by politics, not a legitimate legislative purpose.

The battle over Trump's taxes was joined in July by New York's Democratically controlled legislature. It passed a law, signed by Gov. Andrew Cuomo, that requires tax authorities to provide copies if the state tax returns filed by certain government officials, including the president, if a requesting congressional committee has already asked for the returns from the Treasury Department.

But in August, the judge ordered state authorities to take no action if such a request was made for the state tax returns while the fight over the federal returns is pending.

A hearing is set for Sept. 18 on the government's motion to dismiss the case and the House committee's motion for summary judgment requiring the Treasury Department to turn over the tax returns.