Many federal agencies are bracing for the Trump era — but few are likely to face the powerful backlash that awaits the Bureau of Alcohol, Tobacco, Firearms and Explosives, which pursued an aggressive gun control agenda under President Biden.

The A.T.F., under its Biden-appointed director, Steven M. Dettelbach, has been more proactive on gun control than at any time in its recent history. It has pushed through rules to curb the proliferation of the untraceable homemade firearms known as ghost guns, clamped down on devices that make firearms deadlier and regulated unlicensed firearms sellers who operate at gun shows or online.

That earned praise from gun control groups and drew the enmity of Republicans, including President-elect Donald J. Trump. He is almost certain to pick a proponent of gun rights as director or simply leave the job vacant, as previous presidents have done, leaving the small and embattled bureau rudderless and vulnerable.

But the biggest threat, in the view of Mr. Dettelbach, may come from the Republican-controlled Congress, which is threatening to cut the budget for the federal agency. Its core function is fundamentally apolitical, joining with local enforcement to trace weapons used in crimes and dismantle trafficking rings by providing intelligence and technical assistance.

“People who don’t think that law enforcement, including A.T.F., has anything to do with driving down violent crime are just wrong — it didn’t happen by accident,” said Mr. Dettelbach, sitting in his sprawling, slightly disheveled office at the bureau’s Washington headquarters this month.

“What I am concerned about is that is that people will take their eye off the ball, that they’ll either get complacent or political, or some combination of those things,” said Mr. Dettelbach, a former U.S. attorney in Ohio, whose low-key, Midwestern style belies his willingness to take actions that have elicited political attacks and legal challenges.