The Biden administration has approved the broadest expansion of federal background checks in decades in an attempt to regulate a fast-growing shadow market of weapons sold online that has contributed to gun violence.

Under a rule being released on Thursday, the Bureau of Alcohol, Tobacco, Firearms and Explosives will require anyone “engaged in the business” of selling guns at a profit, including at gun shows and through private sellers, to register as a federally licensed firearms dealer. That means those sellers must run background criminal and mental health checks on potential buyers.

The new regulation, which is likely to face legal challenges, could add as many as 23,000 federal dealers to the 80,000 already regulated by A.T.F., an underfunded division of the Justice Department that already struggles to monitor sellers.

The rule, which drew more than 380,000 public comments, will take effect in a month.

President Biden, repeatedly blocked from enacting universal background checks by Republicans in Congress, is leveraging a provision of the sweeping bipartisan gun control law passed in 2022 to achieve an elusive policy goal that enjoys widespread public support: closing the so-called gun show loophole.

Expanding the number of federal firearms licensees was one of several gun control measures included in an executive order Mr. Biden issued in March 2023 after several mass shootings.

Unlicensed private sellers in many states have been able to legally sell at gun shows, out of their houses and through online platforms without having to submit to the background check system created to prevent sales to children, criminals, domestic abusers, and people with mental illnesses or drug addictions.

Four in 10 illegal gun cases tracked by the bureau from 2017 to 2021 involved such unregulated sales, including thousands from shadow dealers who used legal loopholes to evade background checks, according to an analysis of firearms trafficking released last week.

The purpose of the new rule is twofold, officials said: first, to pull legitimate sellers into the regulatory sunlight and, second, to deprive brokers who knowingly traffic in criminal gun sales of a legal shield provided by the vagaries of federal firearms laws.

Dealers have previously been required to join the federal system only if they derived their chief livelihood from selling weapons. The bar is much lower now — the government has to prove only that they sold guns to “predominantly derive a profit” from their actions.

Failing to register carries a penalty of up to five years in prison and $250,000 in fines.

Vice President Kamala Harris, tapped to lead White House efforts on a gamut of politically charged election-year issues, including gun policy, told reporters in a call on Wednesday that the new regulation addresses “one of the biggest gaps” in the federal background check system.

“This single gap in our federal background check system has caused unimaginable pain and suffering,” Ms. Harris said, who also noted that gun violence was now the leading cause of death among children.

“In the years to come, I do believe countless families and communities will be spared the horror and heartbreak of gun violence by this new rule,” she said.

Steven M. Dettelbach, the A.T.F. director, struck a similar tone. “This is about protecting the lives of innocent, law-abiding Americans as well as the rule of law,” he said.

Mr. Dettelbach, the first permanent director to be approved by the Senate in nearly a decade, has overseen a succession of more modest regulatory moves, including an effort to regulate deadly homemade firearms known as ghost guns.

The administration believes the new regulation is on solid footing, because it is rooted in a newly passed law, rather than a novel interpretation of an existing one. Nonetheless, it is likely to prompt legal fights.

After a preliminary version of the rule was announced last year, Gun Owners of America, a group that has opposed Mr. Biden’s efforts at gun control, called the regulation a “backdoor” universal background check and vowed that its “attorneys will be preparing a lawsuit.”

The announcement comes as the administration has ramped up its efforts to find workarounds to deliver on policy promises to key constituencies, like young voters and communities of color, on issues like gun violence, where Mr. Biden’s priorities have no chance of passing in a divided Congress.

The gun control bill, one of the administration’s most significant policy achievements, has provided the government with several tools to combat a flood of illegal firearms.

The most important, officials said, is a new drug-trafficking charge that is starting to be used in gun cases around the country. Enhanced background check provisions have enabled the Justice Department to stop more than 600 illegal gun purchases by people younger than 21, and stopped straw purchases by third-party buyers that account for roughly 40 percent of illegal gun cases brought by federal prosecutors.

Scores of guns used in crimes have been purchased through the shadow market, increasingly through online marketplaces, like Armslist, a Craigslist for firearms that matches buyers and sellers.

In October 2022, a 19-year-old with a history of mental health issues was denied an AR-15-type rifle at a federally licensed dealer near St. Louis. Shortly thereafter, he bought one through Armslist — this time without a background check — then used it to kill two people and injure several others.