WASHINGTON — In what could be the biggest expansion of federal background checks in decades, the Biden administration is moving to end the controversial “gun-show loophole.”

“This single gap in our federal background check system has caused unimaginable pain and suffering,” said Vice President Kamala Harris in a call with reporters.

On Thursday, the Justice Department will submit a new 466-page regulation to the Federal Register outlining that the Bureau of Alcohol, Tobacco, Firearms and Explosives will require anyone “engaged in the business” of selling guns at a profit to register as a federally licensed firearms dealer and run background criminal and mental health checks on buyers.

The new rules would cover those selling guns not only at gun shows, but also at flea markets and online. In addition, the definition of being “engaged in the business” has been expanded to include those who sell small numbers of firearms. However, exceptions will be maintained for those selling private collections and family heirlooms.

The Justice Department says the new regulation will affect more than 23,000 unlicensed dealers and tens of thousands of gun sales every year.

Republican Sens. John Cornyn, of Texas, and Thom Tillis, of North Carolina, plan to introduce a joint resolution disapproving of the changes.

Attorney General Merrick Garland said of the change in a statement, “Under this regulation, it will not matter if guns are sold on the internet, at a gun show, or at a brick-and-mortar store: if you sell guns predominantly to earn a profit, you must be licensed, and you must conduct background checks.”

He called the regulation “a historic step in the Justice Department’s fight against gun violence.”

In a video posted Thursday on X, President Joe Biden said: “Congress needs to finish the job and pass universal background check legislation now.”

ATF Director Steven Dettelbach said the change is set to take effect in a month. It is likely to face legal challenges, though the administration argues it will hold up in court by using a provision of the sweeping gun control law Congress passed in 2022.

“This final rule does not infringe on anyone’s Second Amendment rights and it will not negatively impact the many law abiding licensed firearms dealers in our nation,” Dettelbach said.

The rule will go into effect 30 days after it is published in the Federal Register.

Advocates have been intensifying calls for the move since the 2022 mass school shooting in Uvalde, Texas, and it comes as the Biden campaign seeks to highlight the administration’s efforts to reduce gun violence. Last month, Harris visited Parkland, Florida, and met families whose loved ones were murdered during the 2018 mass shooting there. In December, the vice president also brought together nearly 100 state legislators from 39 states to launch an initiative that would provide states with additional tools to advance gun safety measures. In September, Biden established the first-ever White House Office of Gun Violence Prevention.

“This is maybe the most impactful change made possible by the 2022 gun safety bill,” said Sen. Chris Murphy, D-Conn., who has been outspoken about the issue since the 2012 shooting at Sandy Hook Elementary.