Critics say the administration is weakening public safety. Proponents say regulations would be where they were before President Joseph R. Biden took office.

The Trump administration is scrapping more than three dozen firearms regulations, abandoning a crackdown on illegal sales, restoring gun rights to some people with mental illness and loosening oversight of private weapons transactions.

The drastic retrenchment at the Bureau of Alcohol, Tobacco, Firearms and Explosives, the federal agency responsible for enforcing the nation’s gun laws, was not entirely unexpected: President Trump campaigned as a champion of gun rights.

In the view of critics and even some A.T.F. veterans, the agency, in closely mirroring the demands made by gun owners and manufacturers to lighten their regulatory burden, is enacting changes at the expense of public safety. The moves, they worry, come as the bureau has already been weakened, with hundreds of its officials diverted to immigration enforcement.

Proponents of the changes point out that some of the reversals would return regulations to what they were only a few years ago, before President Joseph R. Biden took office. After a series of deadly mass shootings, Mr. Biden signed into law gun control measures, ending nearly three decades of gridlock over whether and how to regulate firearms.

The divisiveness illustrates the complicated landscape for gun policy.

“With the Biden regulations that we got and put in place, we advanced the ball,” said Kris Brown, the president of the Brady Campaign to Prevent Gun Violence, one of the country’s biggest gun control organizations.

But the Trump administration’s approach “takes us back 100 years,” she said. “It’s really decimating A.T.F.’s ability to regulate this industry.”