After gains by organized labor under President Biden, a second Trump administration is likely to change course on regulation and enforcement.

Joseph R. Biden Jr. promised to be the most pro-labor president in history. He embraced unions more overtly than his predecessors in either party, and filled his administration with union supporters.

Labor seemed to respond accordingly. Filings for unionization elections spiked to their highest level in a decade, as did union victories. There were breakthroughs at companies like Starbucks and Amazon, and unions prevailed in organizing a major foreign auto plant in the South. A United Automobile Workers walkout yielded substantial contract gains — and images of Mr. Biden joining a picket line.

As Donald J. Trump prepares to retake the White House, labor experts expect the legal landscape for labor to turn sharply in another direction.

Based on Mr. Trump’s first term and his comments during the campaign — including his praise for Tesla’s chief executive, Elon Musk, for what he said was Mr. Musk’s willingness to fire striking workers — these experts say the new administration is likely to bring fewer challenges to employers who fight unions.

“There will be a concerted effort to repeal pro-worker N.L.R.B. precedents,” said Heidi Shierholz, a senior Labor Department official during the Obama administration, referring to the National Labor Relations Board.

Experts like Ms. Shierholz, who is now president of the liberal Economic Policy Institute, said they also expected the Trump administration to ease up on enforcing safety rules, to narrow eligibility for overtime pay and to make it harder for gig workers to gain status as employees.

A Trump transition spokesman did not respond to a request for comment.

Labor unions generally backed Vice President Kamala Harris in the recent election, but a few prominent ones, like the International Brotherhood of Teamsters and the International Association of Fire Fighters, stayed neutral after endorsing Democratic presidential candidates in the past. It is unclear whether that neutrality will yield more influence for these unions in the new administration.

The N.L.R.B. may be the agency that swings sharply most quickly. Its general counsel prosecutes labor law violations, and its five-member board decides questions of legal doctrine, a kind of high court for labor issues.

Mr. Biden fired the agency’s general counsel shortly after taking office and installed a replacement more in sync with his priorities. Mr. Trump is likely to do the same, which would mean ousting Jennifer Abruzzo, who has aggressively wielded the powers of her office on behalf of workers seeking to unionize.

Ms. Abruzzo has sought increased financial remedies for workers who were fired for organizing, and has taken action against companies for holding mandatory meetings that highlight the downsides of unionizing.

If Mr. Trump’s first term is a guide, his appointee as general counsel will probably adopt an approach that makes it harder for union campaigns to succeed. His appointees to the labor board will probably reverse Biden-era rulings on high-profile issues, including one allowing the board to order recognition of a union if the employer makes a fair election impossible, such as by firing workers who seek to unionize.

The Trump labor board will probably also undo a decision that largely struck down the use of gag rules in severance agreements, according to Marshall Babson, a lawyer who works with management and served on the board in the 1980s. These agreements generally prevent workers from describing unsafe working conditions or sexual harassment at their former workplace. And it could make it easier for large companies to avoid recognizing and bargaining with unions formed by employees of contractors or franchises — like Amazon and its delivery drivers, some of whom have recently unionized. All would be consistent with Mr. Trump's earlier approach as president.

Such changes may also come sooner than usual. Presidents have traditionally left labor board members to serve out their terms — which could mean effective Democratic control of the board until at least August 2026. But several experts predicted that Mr. Trump would move to install a Republican majority shortly after taking office, challenging a generally cited legal standard if necessary.

A new direction is also expected at the Department of Labor, which enforces minimum-wage, overtime and worker-safety rules. Under Mr. Biden, the department substantially increased the number of salaried workers who are eligible for overtime pay by raising the cutoff — to about $59,000 next year, from about $35,500 under Mr. Trump. It issued a rule making it likelier that gig workers would be classified as employees rather than independent contractors, entitling them to the federal minimum wage and overtime pay, and stepped up enforcement of child labor violations.

The new administration may rein in overtime eligibility and undo the Biden rule on independent contractors, reflecting Mr. Trump’s first-term approach. It is also likely to tamp down enforcement of child labor rules by the Labor Department, which last year instructed officials to seek large monetary penalties from violators.

“It’s hard to believe that a Trump D.O.L. would be pursuing child labor as aggressively as this administration has,” said Lee Schreter, a lawyer at the firm Littler Mendelson, which represents employers. She said the department had recently asked some of her clients to pay much larger penalties than employers had paid in the past, including during the first Trump administration.

Safety rules could also be affected. Under Mr. Biden, the department’s Occupational Safety and Health Administration proposed requiring employers to protect workers from the health risks of heat exposure by providing drinking water, rest breaks and sufficient cooling indoors. It sought penalties against Amazon for exposing warehouse workers to “ergonomic hazards capable of causing serious physical harm.” (Amazon appealed the citations.)

Ann Rosenthal, a longtime OSHA lawyer in Democratic and Republican administrations, said she expected the Trump administration to delay the heat rule, which it can do with little consequence for years. She said she also expected the agency to curtail investigations of employers in cases where the law gives it discretion.

But experts say less sympathetic regulators won’t necessarily mean a large drop in labor activism, because organizing can develop a momentum of its own: Workers who have seen other workers successfully take on employers often follow suit.

“A big part of what’s been going on is a demonstration effect — that this is something you can do — and that is independent of the legal environment,” said Barry Eidlin, a sociologist who studies labor at McGill University in Montreal.

Workers may simply alter their organizing strategies. For example, graduate students at private universities, who have been unionizing in large numbers over the past few years, might stop filing paperwork to hold union elections under Mr. Trump. They could fear that his labor board would rule that they lacked a federally protected right to unionize, as previous Republican labor boards did.

Instead, they might seek voluntary recognition from their universities, then protest and perhaps even strike until the universities grant it. “There are a lot more people who are going to be saying, ‘What do you mean you’re trying to prevent me from having a union?’” Dr. Eidlin said.

Something similar could happen at companies, said Michael Lotito of Littler Mendelson, an expert on labor relations. He said that unions often pivoted to targeting a company’s reputation when the legal landscape became less favorable, and that these campaigns could be effective.

“A lot of companies make the calculations that we can deal with a union — we can have a strike, negotiations — those are just costs,” Mr. Lotito said. “But reputational risk is what’s gotten their attention. Unions are very good at destroying their reputation.”

Mr. Trump could prompt activism in less direct ways, too. During his first term, his policies seemed to make protest a more mainstream activity, including for tens of thousands of teachers in states like West Virginia and Oklahoma who walked off the job in 2018.

“The Trump presidency increased a sense of the numbers of people on the street that has had knock-on effects for labor,” said Charmaine Chua, a political scientist who studies labor at the University of California, Santa Barbara. “The red-state strikes felt like kind of a symptom of that.”