A federal judge is requiring the head of the Office of Personnel Management (OPM) to testify on Thursday about the mass firing of probationary employees, setting up a legal standoff that could potentially result in the reinstatement of thousands of government workers.

A group of federal unions has alleged that OPM Acting Director Charles Ezell lied in a sworn declaration that his office did not order the firing of probationary employees based on “performance or misconduct,” prompting U.S. District Judge William Alsup to order Ezell to testify in person and under oath in San Francisco on Thursday.

The Trump administration has attempted to push back on the order — arguing in a filing Monday that the testimony raises “fundamental constitutional concerns” — and suggested that Ezell might miss the hearing despite the court’s order.

Judge Alsup late Monday denied their request to cancel the hearing and still wants to directly hear from Ezell, who, as the acting director of OPM, effectively runs the HR department for the federal government.

“The problem here is that Acting Director Ezell submitted a sworn declaration in support of the defendant’s position but now refuses to appear to be cross examined or to be deposed,” Judge Alsup wrote in an order Monday night.

Judge Alsup’s order effectively gives the Trump administration two choices to fight the lawsuit. If Ezell skips the hearing, Judge Alsup said he would consider sanctioning the government and nullifying Ezell’s sworn declaration about the purported legality of the mass firings, a move that would likely mean that the judge pauses the mass firings and potentially reinstates thousands of employees.

Alternatively, if Ezell does testify on Thursday, he’s likely to face tough questions about his sworn statements related to the firings.

The plaintiffs allege that on Feb. 13, Ezell convened a phone call with the heads of federal agencies to direct them to terminate thousands of federal employees and “falsely state that the terminations are for performance reasons.”

In a sworn declaration last month, Ezell denied directing the terminations based on performance reasons, instead arguing that OPM only issued guidance to individual agencies about the need for probationary workers to “demonstrate why it is in the public interest” for the government to continue to employ them.

“OPM did not direct agencies to terminate any particular probationary employees based on performance or misconduct, and did not create a ‘mass termination program,’ as the plaintiffs in this matter described it,” Ezell wrote.

The groups challenging the firings in court say that was a lie, and Judge Alsup appeared inclined to agree during a court hearing last month.

“How could so much of the work force be amputated suddenly overnight? It’s so irregular and so widespread and so aberrant from the history of our country,” Judge Alsup said. “How could that all happen with each agency deciding on its own to do something so aberrational?”

“I don’t believe it,” said the judge. “I believe they were directed or ordered to do so by OPM in that telephone call. That’s the way the evidence points.”

The allegations about the mass firings comes as the Trump administration faces increased scrutiny about the role of the Department of Government Efficiency in reducing the size of the federal government. During a cabinet meeting last week, Trump told the heads of the federal agencies that they are in charge of making cuts to their own departments, rather than Elon Musk and DOGE.