A labor union that represents federal workers filed a lawsuit to challenge a law that caps government debt, as President Joe Biden and congressional leaders navigate a standoff over the nation’s borrowing limit.

The National Association of Government Employees asked a federal court Monday to block Treasury Secretary Janet L. Yellen from complying with the borrowing limit set by the law, arguing that the statute is unconstitutional. The lawsuit also asks the court to stop Biden and Yellen from defunding any federal government operations unless Congress specifically directs to do so.

The litigation dovetails with arguments that the Constitution’s 14th Amendment, which states that “the validity of the public debt of the United States … shall not be questioned,” could be leaned on as a way of circumventing the debt limit showdown.

That choice has received different reactions from presidents in the past, with Bill Clinton endorsing the idea years ago but Barack Obama throwing cold water on the option while in office.

The federal government could run out of borrowing room and be unable to pay all its bills on time, and that “x date” could be as soon as June 1, according to Yellen, who warned Monday of “economic catastrophe” and “financial chaos” if action isn’t taken in time.

The union’s lawsuit, filed in the U.S. District Court for the District of Massachusetts, says the debt limit statute allows the president to cancel government spending that’s been approved by Congress, and that violates the separation of powers.

The lawsuit states that the union does not seek to challenge the “controversial proposition” that Congress can limit the country’s debt. But it contends that “Congress may not do so without at least setting the order and priority of payments once that limit is reached, instead of leaving it to the President to do so.”

“Nothing in the Constitution or any judicial decision interpreting the Constitution allows Congress to leave unchecked discretion to the President to exercise the spending power vested in the legislative branch by canceling, suspending, or refusing to carry out spending already approved by Congress,” the lawsuit states.

Union members are now under imminent risk of layoff or furlough, the lawsuit states. The union says it is the exclusive bargaining representative of nearly 75,000 federal employees.

Yellen, who is named as a defendant in the union lawsuit, has given an icy reception to arguments about the 14th Amendment.

“We should not get to the point where we need to consider whether the president can go on issuing debt. This would be a constitutional crisis,” Yellen said Sunday on ABC News.

Republican Sen. James Lankford of Oklahoma agreed with that analysis on the same show.

“It’s certainly not a good option, and she rightfully said it would be a constitutional crisis,” Lankford said.

For his part, Biden said in a recent interview that he’s not ready to invoke the 14th Amendment to deal with the debt limit issue. But he also did not take the idea off the table.

In 2011, Clinton said he would use the 14th Amendment “without hesitation, and force the courts to stop me.”

While in office, Obama did not throw his weight behind the 14th Amendment idea. He said it would be “tied up in litigation for a long time,” even if it were constitutional.