When President Biden took office, he swiftly canceled an executive order his predecessor Donald J. Trump had issued that could have enabled Mr. Trump to fire tens of thousands of federal workers and replace them with loyalists. But Democrats never succeeded in enacting legislation to strengthen protections for the civil service system as a matter of law.

Now, with Mr. Trump seemingly poised to win the G.O.P. nomination again, the Biden administration is instead trying to effectively Trump-proof the civil service with a new regulation.

On Friday, the White House proposed a new rule that would make it more onerous to reinstate Mr. Trump’s old executive order if Mr. Trump or a like-minded Republican wins the 2024 election.

But Trump allies who would most likely have senior roles in any second Trump administration shrugged off the proposed Biden rule, saying they could simply use the same rule-making process to roll back the new regulation and then proceed. Legal experts agreed.

The proposed rule addresses the move Mr. Trump tried to make late in his presidency by issuing an executive order known in shorthand as Schedule F. It would have empowered his administration to strip job protections from many career federal employees — who are supposed to be hired based on merit and cannot be arbitrarily fired. While the order said agencies should not hire or fire Schedule F employees based on political affiliation, it effectively would have made these employees more like political appointees who can be fired at will.

Career civil servants include professional staff across the government who stay on when the presidency changes hands. They vary widely, including law enforcement officers and technical experts at agencies that Congress created to make rules aimed at ensuring the air and water are clean and food, drugs and consumer products are safe.

Mr. Trump and senior advisers on his team came to believe that career officials who raised objections to their policies on legal or practical grounds — including some of their disputed immigration plans — were deliberately sabotaging their agenda. Portraying federal employees as unaccountable bureaucrats, the Trump team has argued that removing job protections for those who have any influence over policymaking is justified because it is too difficult to fire them.

Critics saw the move as a throwback to the corrupt 19th-century patronage system, when all federal jobs were partisan spoils rather than based on merit. Congress ended that system with a series of civil-service laws dating back to the Pendleton Act of 1883. Everett Kelley, national president of the American Federation of Government Employees, described Schedule F as “the most profound undermining of the civil service in our lifetimes.”

The legality of Schedule F was never tested because Mr. Biden revoked the order before any federal workers were reclassified. But Mr. Trump has vowed to reinstate it if he returns to office in 2025 — and his motivations, now, are openly vengeful. He has boasted that he will purge a federal bureaucracy that he has disparaged as a “deep state” filled by “villains” like globalists, Marxists and a “sick political class that hates our country.”

The proposed new rule was unveiled by the White House’s Office of Personnel Management in a lengthy filing for the Federal Register on Friday. It would allow workers to keep their existing job protections, such as a right to appeal any firing or reassignment, even if their positions were reclassified. It would also tighten the definition of what types of positions can be exempted from civil service job protections, limiting it to non-career political appointees who are expected to turn over when a presidency ends.

The regulatory proposal argued that maintaining protections for career civil servants enhances the functioning of American democracy because such federal workers have institutional memory, subject matter expertise and technical knowledge “that incoming political appointees may lack.” They should be free to disagree with their leaders — short of defying lawful orders — without fear of reprisal, the proposed rule states.

The public will now have 60 days to comment on the proposed rule, but the Biden administration expects to complete it by early 2024.

A spokesman for the Trump campaign did not respond to an email seeking comment on Mr. Biden’s effort. Biden officials and people supportive of their plan are projecting optimism about the significance of the new regulation to bolster protections for the civil service. Among them is Rob Shriver, the deputy director of the Office of Personnel Management, essentially the government’s human resources department.

“Our proposed regulation is strong and based in law and has a strong rationale,” Mr. Shriver said. “Anyone who wants to explore a change in policy would have work to do,” he added. “They’d have to go through the same administrative rule-making process and make sure that their policy is grounded in the law.”

Mr. Trump’s allies have been aware of the proposed rule since the spring, when the Biden administration cited it on a government website as part of its 2023 regulatory agenda. Trump allies say they don’t expect it to do much more than delay by a number of months their renewal of Schedule F if Mr. Trump wins back the presidency.

James Sherk, the former Trump administration official who came up with the idea for Schedule F, defended the order and said that reimposing it would not be difficult despite the new rule.

“The Biden administration can, if they want, make removing intransigent or poorly performing senior bureaucrats harder on themselves,” said Mr. Sherk, who now works at the America First Policy Institute, a think tank stocked heavily with former Trump officials. “The next administration can just as easily rescind those restrictions. With regards to reissuing Schedule F, this proposed rule would be a speed bump, but nothing more.” Another fervent supporter of Schedule F is Russell T. Vought, the president of the Center for Renewing America, a think tank with close ties to the former president. In the Trump administration, Mr. Vought had been the director of the Office of Management and Budget. He proposed reassigning nearly 90 percent of his agency’s staff as Schedule F employees, making them vulnerable to being summarily fired if he deemed them obstructive to the president’s agenda.

That threat was never acted upon — Mr. Trump issued the Schedule F order in October 2020, shortly before losing re-election — but Biden administration officials say that career civil servants are still living with the hangover from what nearly happened and are anxious about the prospect of Schedule F returning.

Jason Miller, a senior official in Mr. Biden’s Office of Management and Budget who has worked on the new rule, said in an interview that Mr. Trump’s Schedule F order “exposed the fragility of the existing system — the system that has been in place for 140 years to ensure we have a dedicated nonpartisan civil service.”

Mr. Miller said the impact of Schedule F “is still felt to this day.” He added, “We have carried that with us. It is not just here in O.M.B. It is across federal agencies.”

Mr. Vought, however, said Schedule F was about removing poor performers, and characterized the proposed regulation as little impediment to reviving the idea.

“This expected move by the Biden administration to forestall accountability within the bureaucracy against poor performers merely reinforces what we already knew — Schedule F rests on a sound legal foundation, is going to succeed spectacularly and the only chance to stop it is to install procedural roadblocks,” he said.

Even if Mr. Trump unexpectedly loses the Republican nomination, there’s a good chance that whoever defeats him will also plan to dismantle the administrative state. Schedule F has swiftly become doctrine across a large swath of the G.O.P., and two of Mr. Trump’s leading rivals are indicating they want to go even further than he does.

“On bureaucracy, you know, we’re going to have all these deep-state people, you know, we’re going to start slitting throats on Day 1 and be ready to go,” said Gov. Ron DeSantis of Florida at an event in New Hampshire in July.

On Wednesday, the businessman Vivek Ramaswamy outlined an even more radical plan than Mr. Trump’s for dismantling much of the government. Mr. Ramaswamy said he would shut down multiple federal agencies and fire 75 percent of the federal work force, although both the legal and practical substance undergirding his attention-seeking proposal appeared thin.

“I would not view the efforts to protect the integrity of the professional civil service as just antidotes to Trump,” said Representative Jamie Raskin of Maryland, the ranking Democrat on the House Committee on Oversight and Reform, which has jurisdiction over the federal civil service. “I see them as authoritarianism repellents, generally.”

Democrats had initially tried to change federal law to prevent any return of Schedule F, but opposition by Republicans — where Senate rules allow a minority of 40 lawmakers to block most legislation — thwarted the effort.

When the House was still controlled by Democrats in the first two years of Mr. Biden’s presidency, it attached a measure strengthening protections for the merit-based civil service system as an amendment to a “must-pass” annual defense bill in 2022. But Republican opposition kept it off the Senate version and then forced Democrats to drop it when the two versions were reconciled.

Democrats used their control of the House in Mr. Biden’s first two years to pass proposed reforms in response to the ways in which Mr. Trump’s presidency flouted norms. Other ideas Democrats proposed included making it harder for a president to offer or bestow pardons in situations that raise suspicion of corruption, to refuse to respond to oversight subpoenas and to take outside payments while in office. The House passed a bill that combined those and other ideas in December 2021. But Republicans almost uniformly opposed such measures, portraying them as partisan attacks on Mr. Trump, and the Senate’s filibuster rule meant they had the power to block them from becoming law. And Mr. Biden did not make enacting post-Trump reforms a bully-pulpit focus.

Max Stier, president of the Partnership for Public Service, a nonpartisan group that seeks to make government more effective, has been working with the Biden administration on this and other proposals to bolster the civil service. He said he understands the vulnerability of the new proposed rule to being overturned, but he said it would make reimposing Schedule F even more vulnerable to legal challenges than it was when Mr. Trump first issued the order.