The Biden administration approved the largest expansion of background checks for gun purchases in decades on Thursday.

Why it matters: The new rule, which is almost certain to be challenged in the court, would require thousands of gun sellers to register as federally licensed firearms dealers. They would then have to run criminal and mental health background checks on potential buyers.

How it works: The rule announced Thursday is intended to close the "gun show loophole" and the related "online sale loophole" by forcing anyone who sells a firearm for profit to register with the federal government.

• Due to those loopholes, around 20,000 dealers currently operate without licenses and are thus not required to conduct background checks, per AP.
• President Biden said in a statement that the new rules will "keep guns out of the hands of domestic abusers and felons."

The big picture: The new rule marks the largest expansion of background checks for firearms purchases since the 1993 Brady Bill.

• However — like the Brady Bill and several other laws meant to decrease the U.S.' abnormally high levels of gun violence — the new rule will likely be challenged by firearm interest groups and could be pared back by the courts.
• With Democrats blocked by Republicans from passing legislation requiring universal background checks, Biden has instead leveraged provisions in a bipartisan gun control law passed in 2022.

What they're saying: Biden called on Congress to "finish the job" and pass universal background checks legislation.

• "I've spent hours with families who've lost loved ones to gun violence. They all have the same message: 'Do something,'" he said. "Today, my Administration is taking action to make sure fewer guns are sold without background checks."
• Former U.S. Rep. Gabby Giffords, who was severely injured in a 2011 shooting, said in a statement Thursday that the loopholes the rule seeks to close allow "anyone intent on doing harm to buy a gun."
• "This new rule will make sure fewer people slip through the cracks, and we're all safer for it," she said, adding that "we are closer to universal background checks than ever."

Yes, but: Sens. John Cornyn (R-Texas) and Thom Tillis (R-N.C.), the lead Republican negotiators on the bipartisan gun control law, plan to introduce a resolution of disapproval under the Congressional Review Act to overturn the new rule, a Cornyn spokesperson said Thursday.

• The senators believe the rule is unconstitutional, the spokesperson said, adding that other Senate Republicans have signed on in support.

Between the lines: The resolution is a longshot in the Democrat-held Senate.

• If it does pass the House and Senate, Biden could issue a veto, which can only be overridden by two-thirds of the members of each chamber.

Zoom out: Around 40% of illegal gun cases in the U.S. investigated by the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) came from unlicensed firearm dealing by private persons, according to new ATF analysis released last week.

• Many polls have shown that expanding background checks has widespread public support.

Go deeper: Nearly a quarter of K-12 teachers faced a gun lockdown last year

Editor's note: This story has been updated with comment from former Rep. Gabby Giffords.