WITH THE stroke of a pen on Jan. 19, the day before President Biden took office, employees of the federal government’s main deportation agency were empowered to thwart the new administration’s policies. The union representing Immigration and Customs Enforcement agents, a longtime ally of former president Donald Trump, was handed what amounts to a veto over the White House thanks to the machinations of one man: Ken Cuccinelli, then-acting deputy secretary at the Department of Homeland Security.

Mr. Cuccinelli, an immigration hard-liner, schemed during the outgoing administration’s final days to fix in place its deportation policies and stymie those of Mr. Biden. A former Republican attorney general of Virginia once fond of bashing Democrats’ pro-labor policies, he signed a deal with the union representing ICE agents and employees that may be among the most pro-labor agreements ever reached by the federal government.

In a series of memorandums of understanding reported by the New York Times, Mr. Cuccinelli, whose own appointment had been deemed legally questionable, agreed with ICE’s union that the federal government would be forbidden from making any “modifications whatsoever concerning the policies, hours, functions, alternate work schedules, resources, tools, compensation and the like” affecting the agency and roughly 7,500 agents and employees — without the explicit written consent of union officials.

Mr. Cuccinelli’s intervention, which also included signing pro-union measures that may cost taxpayers millions of dollars annually, was reported by an anonymous whistleblower whose lawyer, David Z. Seide, called it a shocking abuse of authority. It is also an anti-democratic attempt, by an unelected bureaucrat, to grant a public employee union stunning power to pursue its own immigration agenda and ignore that of a duly elected president.

The Biden administration agenda includes the new president’s campaign promise to suspend most deportations during his first 100 days in office and broadly reprioritize them thereafter, to focus on migrants guilty of violent crimes and those deemed a threat to national security. No doubt, some or many ICE agents privately disagree with that policy. Under the president’s executive authority, however, it is — or should be — their duty to implement it. Now, if the Biden administration is to challenge the agreements signed by Mr. Cuccinelli and the ICE union, it may face a protracted fight in the Federal Labor Relations Authority, to which the union is entitled to appeal.

For now, also thanks to Mr. Cuccinelli’s mischief, deportation policy is unsettled. On Jan. 8, he signed an agreement with Texas committing the federal government to give that state 180 days’ notice before shifting the federal government’s deportation policy — an extraordinary concession and perhaps one at odds with the Constitution’s supremacy clause. Based on that, Texas sued to block the new administration’s 100-day pause on deportations, and a Trump-appointed judge granted a nationwide restraining order blocking it for the time being. In the past week, ICE has deported several hundred undocumented migrants to Central America and dozens to Haiti, including a woman who had agreed to testify as a witness against the gunman accused of killing 23 people in the 2019 massacre at an El Paso Walmart, the Associated Press reported.

Mr. Cuccinelli is now out of government; the damage he has done may endure.