WASHINGTON — In their aggressive bid to upend the federal government, Elon Musk and his lieutenants have, perhaps unintentionally, driven thousands of federal workers into the arms of one of the biggest enemies of their sweeping project.

Organized labor.

Unions that represent federal government workers are reporting significant increases in signups since President Trump took office and empowered Musk’s “Department of Government Efficiency” to drastically slash the federal workforce.

The American Federation of Government Employees, the largest union representing federal workers, added 5,575 new members in the first six days of February alone, according to union spokesperson Tim Kaufman. By comparison, the union gained 14,996 new members during all of 2024.

Currently, AFGE has 319,233 active members — a record, said Kaufman, exceeding its previous high-water mark in 2018, during the first Trump administration.

At another large federal workers union, the National Federation of Federal Employees, membership is “going through the roof,” said union president Randy Erwin.

“Our phones have been ringing off the hook from our members, from the media, but also from people who are unrepresented who want union representation for the first time,” Erwin said. “Typically, we get a handful of these per month. We might be getting 10 per day.”

Erwin did not have available statistics from every federal agency where his union is active, but did say the Forest Service, whose workers the union represents, has seen a 20 percent increase in union membership over a two week period.

“This is at least 20 to 25 times the normal action,” he said. “People are signing up in droves.”

It is not hard to understand why. Trump and Musk have quickly and bluntly made clear to the vast majority of the nation’s roughly 3 million federal workers that their jobs are at risk, a threat union leaders describe as unprecedented.

“If you don’t have union representation,” said Erwin, “you might as well consider yourself an at-will employee under this administration.”

Less than three weeks into the new administration, DOGE has already effectively crippled one federal agency, the US Agency for International Development, where the administration is trying to fire all but 290 of its 10,000 workers. Employees at other federal agencies who work on projects Trump and Musk do not favor have been abruptly put on “administrative leave,” including 160 employees at the Environmental Protection Agency’s environmental justice division. This could represent merely an opening salvo; before the inauguration, Musk talked about cutting the federal workforce by 75 percent.

The biggest development so far, and perhaps the most ominous, came in the form of an email that landed on Jan. 28 in the inboxes of 2 million federal workers with the cryptic subject line “Fork in the Road.” It gave federal workers the option to remain in their jobs — but without “full assurance regarding the certainty of your position or agency” — or to accept what is essentially a buyout, getting paid through September without having to work.

The initial deadline for employees to decide was Feb. 6., but it was halted after a federal district judge in Massachusetts, George O’ Toole Jr., intervened — in response to a lawsuit brought by the American Federation of Government Employees and two other major unions representing federal workers. On Monday, O’Toole will hear arguments from the unions arguing that the buyout offer is unlawful and should not be allowed to proceed. (Unions have urged employees not to take the deal, arguing it is not backed by funding or any real commitment, though multiple outlets reported on Friday that the White House said 65,000 people had accepted.)

The battle over the buyout ultimatum underscores the crucial role for federal employee unions in countering the maneuvers of Trump and Musk. With Democrats sidelined in Washington, organized labor will likely be the key player in lawsuits over the administration’s plans. They will also be marshaling their membership to galvanize elected officials, and public opinion, against the deep government cuts. And they’ll be looking to continue expanding membership, given federal employees are not required to join unions.

Another reflection of labor’s engagement came late on Friday, when a federal judge in the District of Columbia halted the administration’s layoff plans at USAID in response to legal action from public sector unions.

This moment puts public sector unions “at the front lines not just of defending their negotiated contracts and rights but defending the rule of law and democracy itself,” said Jennifer Sherer, a labor advocate who analyzes labor policy at the left-leaning Economic Policy Institute.

“It’s not surprising that folks are looking to unions for accurate and timely guidance on the directives that they’re being given,” she added.

Representative Don Beyer, a Virginia Democrat whose district has the second-largest share of federal workers in the nation, said their unions “are going to play a vital role” in responding to the DOGE onslaught.

After meeting with the heads of major public employee unions this week, Beyer said that, “On the one hand, they’re upset and worried for their people. On the other, they seem determined, angry, and ready for the fight.”

The task for unions comes at a complex political moment for organized labor. Trump won in 2024 with more organized labor support than any Republican candidate in generations and has pledged workers would have influence in his administration. His pick for labor secretary, former representative Lori Chavez-DeRemer, was a rare GOP lawmaker who backed pro-union legislation in Congress; her confirmation hearing is scheduled for next week. Her record stands in contrast to Musk himself, who has long been an opponent of organized labor at his companies.

At the end of 2023, roughly 1 million federal workers were unionized, according to the American Federation of Government Employees. Public sector unions represent a workforce that Trump and his allies have long antagonized as the “swamp” or the “deep state,” building on decades of Republican animosity toward union representation of government workplaces. In the 2024 campaign, the Project 2025 policy blueprint drafted by Trump allies — which is increasingly being carried out now — asserted that government employee unions were incompatible with democracy and suggested Congress consider banning them.

In 2018, foes of those unions notched a major victory when the Supreme Court ruled that government employees did not have to join unions, nor did they have to pay fees to them — despite enjoying the benefits of contracts they negotiated. While the ruling was more directly applicable to state and local government workers, given the differences with federal labor law, Sherer said it had the overall effect of weakening the voice of public sector unions.

Regardless, DOGE’s assault on the federal government may revitalize the public sector unions that Trump and conservatives want to marginalize. Online forums frequented by federal workers, like Reddit, are filled with posts from users asking how to join.

Beyer joked there’s a touch of irony that people are turning to unions at a dire moment. While the speed and questionable legality of DOGE’s moves have been surprising, the intent of the incoming Trump administration has long been clear when it comes to the federal workforce.

The newly confirmed Office of Management and Budget director, Russell Vought, has said he wants to traumatize federal workers. “When they wake up in the morning, we want them to not want to go to work because they are increasingly viewed as the villains,” he said in 2023.

But Erwin, of the National Federation of Federal Employees, said, “I never give someone a hard time about coming to the realization.”

“If you’re ready to be a member now,” he said, “good, glad you saw the light.”

Sam Brodey can be reached at sam.brodey@globe.com. Follow him @sambrodey.