A union is pushing back against the White House plan to issue tax refunds during the government shutdown, a move that would to return Internal Revenue Service employees to work without pay.

The administration said the tax refunds would alleviate the pain of the shutdown felt by nonfederal workers, but a complaint filed by the union could blow up that plan.

The National Treasury Employees Union, which also represents workers at several other affected federal agencies including Customs and Border Protection, filed a complaint in U.S. District Court against acting White House chief of staff Mick Mulvaney that claims many of its members are unconstitutionally being forced to work without pay during the government shutdown.

The complaint asks a federal judge to block the White House’s most recent guidance for operations during the partial shutdown, which includes actions, like processing tax returns and refunding taxpayers, that Vice President Mike Pence said on Monday were aimed at diminishing the pain of the partial shutdown for nonfederal workers.

“Tax refunds will go out,” administration official Russell Vought, who’s acting in Mulvaney’s stead as Office of Management and Budget director, told reporters during the briefing.

The union's lawsuit claims that the definition of public safety being used by the White House is far too broad and encompasses people whose services involve only the ongoing, regular functions of government and not anything that would involve an imminent threat to life or property. Generally only government functions and employees essential to protecting national security or property are deemed essential in a shutdown.

"While a case can certainly be made that some federal employees, such as Customs and Border Protection officers and others, are protecting human life and property, that line of reasoning gets quite shaky when applied to thousands of IRS employees being called back in order to process tax refunds,” the union’s National President Tony Reardon said.

The union argues the Trump administration is abusing a loophole in the law. In the request for an injunction to block those guidances, it alleges that the directions are unconstitutional because they require the government to spend funds that Congress did not approve and force employees to work without pay in a situation where they have been considered nonessential in the past.

The lawsuit is based on the Anti-Deficiency Act, a law that says that federal agencies cannot spend money that has not been appropriated to them. This includes allowing federal workers to appear on the job, which would obligate the government to pay them. The law is what technically causes most of the government to shut down during budget impasses, such as the one that began on Dec. 22.

The act has long been interpreted to mean that federal workers whose job involves public safety are exempted and must show up even if the government is not currently paying them.

“If employees are working, they must be paid — and if there is not money to pay them, then they should not be working,” said Reardon.

In most government shutdowns, federal workers are paid what they would have gotten once the government reopens. According to the Senate Appropriations Committee, there are currently 450,000 federal workers who are working under this exemption, such as those at the FBI and Federal Bureau of Prisons.

A White House spokesperson could not be reached for comment.

Barring an 11th hour breakthrough in the stalemate between President Trump and congressional Democrats, most if not all of the approximately 800,000 federal employees affected by the partial shutdown are due to miss their paycheck on Friday.