WASHINGTON - A proposed regulation announced Thursday by the Biden administration would require thousands of gun sellers to register with the federal government and conduct background checks of their customers.

A small part of the bipartisan gun law Congress passed last year broadened the definition of a “gun dealer” as a way of reducing the number of sales that don’t go through the background check system.

President Joe Biden ordered the Justice Department earlier this year to write the regulation so the government could get “as close as we can to universal background checks without new legislation.”

Only those who are “engaged in the business” of firearms have been required to register as federal firearms licensees, while people who don’t regularly sell guns, such as hobbyists, have been exempt.

In a statement on Thursday, U.S. Attorney General Merrick Garland said the proposal “implements Congress’s mandate to expand the definition of who must obtain a license and conduct a background check before selling firearms.”

The lack of background checks for so-called “private” transactions has been called the “gun show loophole,” though in recent years there have probably more such sales online than at gun shows. Background checks prevent gun sales to people who aren’t allowed to own guns under federal law, such as people convicted of violent crimes.

Steven Dettelbach, director of the Bureau of Alcohol, Tobacco, Firearms and Explosives, said an “increasing number” of people have been selling firearms for profit without registering.

“Instead, they have sought to make money through the off-book, illicit sale of firearms,” Dettelbach said in a statement Thursday. “These activities undermine the law, endanger public safety, create significant burdens on law enforcement, and are unfair to the many licensed dealers who make considerable efforts to follow the law.”

Under the new definition, a person would be said to be “engaged in the business” of firearms if their principal aim is to profit, “as opposed to other intents, such as improving or liquidating a personal firearms collection,” the law says. At the time it was drafted, it wasn’t clear how many people would be affected, since it would be up to the ATF to put it through regulation.

According to the text of the ATF’s proposed rulemaking, the broader definition of a gun seller would affect between 24,540 and 328,296 unlicensed dealers. There were 136,563 active federal firearms licensees in 2022, so the ATF is estimating that the regulation could greatly increase their ranks.

Sen. John Cornyn (R-Texas), one of the authors of the Bipartisan Safer Communities Act, told HuffPost earlier this year that the law’s expanded meaning of “engaged in the business” would basically fulfill Democrats’ goal of expanding background checks, though Cornyn also bristled at Biden’s push for the eventual regulation to get as close to universal background checks as possible. Experts have said the change would not encompass all private gun sales, and Democrats have said they still want to change the law to require universal checks.

Mark Collins, director of federal policy for Brady, a gun control group, said the new regulation would not be tantamount to universal background checks.

“In terms of whether or not it gets us to universal, I think it’s a big step closer to it,” Collins said. “But I would be hesitant to say that it gets us to complete universal without Congress acting.”

Under the proposed rule, a person would be considered “engaged” in the firearm business if they meet certain new criteria, including if they sell someone a gun while telling the buyer more are available; if they repeatedly purchase firearms for sale within 30 days; or if they’re reselling guns in their original packaging.

The public has 90 days to comment on the regulation before the government can begin to finalize it.

A bipartisan group of senators wrote the 2022 law in response to multiple mass shootings. Its main provisions enhanced background checks for gun buyers under 21, created grants for states to enact “red flag” laws, and provided funds for school safety and mental health services.

Sen. Chris Murphy (D-Conn.), one of the law’s main Democratic authors, praised the proposed regulation on Thursday.

“For too long, the ambiguous definition of what it means to be ‘engaged in the business’ allowed thousands of unlicensed dealers to sell guns without background checks,” Murphy said in a statement, adding that the change will “significantly increase the number of background checks performed and ensure more guns stay out of dangerous hands.”