WASHINGTON — Employees of the Environmental Protection Agency have been calling their senators to urge them to vote on Friday against the confirmation of Scott Pruitt, President Donald Trump's contentious nominee to run the agency, a remarkable display of activism and defiance that presages turbulent times ahead for the EPA.

Many of the scientists, environmental lawyers and policy experts who work in EPA offices around the country say the calls are a last resort for workers who fear a nominee selected to run an agency he has made a career out of fighting — by a president who has vowed to "get rid of" it.

"Mr. Pruitt's background speaks for itself, and it comes on top of what the president wants to do to EPA," said John O'Grady, a biochemist at the agency since the first Bush administration and president of the union representing the EPA's 15,000 employees nationwide.

Nicole Cantello, an EPA lawyer who heads the union in the Chicago area, said: "It seems like Trump and Pruitt want a complete reversal of what EPA has done. I don't know if there's any other agency that's been so reviled. So it's in our interests to do this."

The union has sent emails and posted Facebook and Twitter messages urging members to make the calls.

"It is rare," said James A. Thurber, the director of the Center for Congressional and Presidential Studies at American University. "I can't think of any other time when people in the bureaucracy have done this."

The campaign is not likely to succeed. Before Friday's vote, two Democratic senators, Heidi Heitkamp of North Dakota and Joe Manchin III of West Virginia, announced that they would vote for Pruitt's confirmation, and only one Republican, Sen. Susan Collins of Maine, has said she will oppose him.

But because Civil Service rules make it difficult to fire federal workers, the show of defiance indicates that Pruitt will face strong internal opposition to many of his promised efforts to curtail EPA activities and influence.

"What it means is that it's going to be a bloodbath when Pruitt gets in there," said Christine Todd Whitman, a former Republican governor of New Jersey and the EPA administrator during the first term of President George W. Bush.

Whitman predicted a standoff between career employees and their politically appointed bosses, noting that Pruitt will be blocked by legal Civil Service protections from immediately firing longtime employees but would likely be able to retaliate against them in other ways, such as shifting them to different jobs.

The showdown could embolden the White House and Congress to change federal Civil Service laws.

"The Civil Service is supposed to be a class of experts implementing policy, regardless of politics," said Myron Ebell, a fellow at the free-market Competitive Enterprise Institute, who led Trump's environmental transition team. "If they have now become a special interest group pleading their own agenda, then it is probably time to look at reforming the Civil Service laws."

The revolt has also angered supporters of Pruitt.

"There clearly has been an organized effort to demonize Pruitt and I think that's unfair and unfortunate," said Jeffrey Holmstead, a senior EPA official in the George W. Bush administration who has been mentioned as a possible deputy to Pruitt. "I don't remember, in my time, anything like this. But I think that anyone Trump nominated would be targeted."

"We know that he'll dismantle Clean Power Plan and the Waters of the U.S. rule, but he's not going to go in there and start firing people," said Holmstead, referring to Obama regulations on climate change and water pollution.

Pruitt, the attorney general of Oklahoma, has sued the EPA at least 14 times, often in concert with the nation's largest fossil fuel companies, to block major environmental regulations. He has questioned human-caused global warming and is a key architect of the national legal effort to dismantle former President Barack Obama's climate change policies.

He has harshly criticized the role of the federal agency, saying much of its authority should be dissolved and left to the states. Pruitt's legal views on environmental protection broadly, and the role of the EPA specifically, appear to line up with Trump's campaign claim that "environmental protection, what they do is a disgrace."

Within days of Pruitt's swearing-in, Trump is expected to sign one or more executive orders aimed at undoing Obama's climate change regulations, and possibly to begin dismantling some EPA offices and programs, according to people familiar with the White House's plans.

While it will be impossible to undo most major rules or programs that quickly, the presidential signatures would authorize Pruitt to cut existing environmental regulations — and, eventually, the jobs of many of the people who enforce them.

Cantello said most of her career at the EPA has been focused on water protection, particularly on cleaning pollution in the Great Lakes.

"I'm afraid all the work I've done will be abandoned," she said.

Cantello and other longtime agency employees said that while they sometimes chafed under the administration of George W. Bush, who sought to loosen some environmental rules, they did not openly rebel against it — nor, they said, did they fear that Bush and his appointees wanted to eliminate the agency.

"I've been here for 30 years, and I've never called my senator about a nominee before," said an EPA employee in North Carolina who spoke on the condition of anonymity out of fear losing her job.

The calls to senators come on top of an anti-Pruitt protest last week by Chicago EPA employees, and agency workers say that if Pruitt is confirmed, they intend to amplify their resistance to him, taking their case to the American public.

"At this point, it's just, 'call your senator,'" O'Grady, the union president, said. "We plan on more demonstrations, more rallies. I think you will see the employees' union reaching out to NGOs and having alliances with them," he added, referring to nongovernmental organizations. "We're looking at working with PR firms."

The White House and EPA did not respond to emailed questions about the employees' campaign.

The EPA emerged as a Republican political target during the Obama administration, after Obama turned to the agency to muscle through an environmental agenda that could not get through Congress.

While Trump campaigned on slashing Obama-era rules on climate change and waterways, his efforts might also be thwarted by Congress. But the EPA is likely to be at the center of his anti-regulatory agenda.

Experts say it is not surprising that liberal and environmental groups, such as the Sierra Club, have campaigned against Pruitt. More than 700 former EPA employees have signed a letter to senators opposing his confirmation.

The Center for Media and Democracy, a left-leaning group, successfully sued the Oklahoma attorney general's office to release about 3,000 of Pruitt's emails, which they say could reveal more about his close ties with fossil fuel companies. An Oklahoma judge ruled Thursday afternoon that the emails must be released but gave the attorney general's office until Tuesday to comply, long enough to avoid roiling the confirmation vote unless Democrats can persuade Senate Republicans to hold off.

But former EPA officials said the open rebellion by current employees is extraordinary, especially considering that their resistance could backfire once Pruitt arrives on the job.

"EPA staff are pretty careful. They're risk-averse," said Judith Enck, who left the agency last month. "If people are saying and doing things like this, it's because they're really concerned."

Sen. Rob Portman, R-Ohio, said Wednesday that after his office had received dozens of calls from people both opposing and supporting Pruitt's nomination, including EPA employees, he had not yet decided whether to vote for him.

"I do have concerns about the Great Lakes," he said.

O'Grady said that he expected the calls to continue through Friday's vote.