Senators struck a hard-fought compromise Wednesday that could expand US background checks to all commercial firearms sales, which may result in the most ambitious change to gun laws since 1994.
The deal comes four months after a massacre at Sandy Hook Elementary School in Connecticut, which left 20 young children and six adults slaughtered, and took America's epidemic of gun violence to an alarming new level.
"We have an agreement... on an amendment to prevent criminals and the mentally ill and insane from getting firearms," Democrat Joe Manchin, from the pro-gun state of West Virginia, said as he unveiled the measure.
The amendment would see background checks -- the core of President Barack Obama's gun control push -- expanded to include all sales at gun shows and on the Internet, said Manchin and Republican Pat Toomey, the deal's architects.
Despite falling short of the president's proposal for universal background checks, Obama noted the "significant bipartisan progress" of the compromise but warned that much work remains.
"Congress needs to finish the job," the president said, adding that Senate approval would require the ability to overcome obstruction by Republicans opposed to gun control measures.
And "any bill still has to clear the House," Obama added.
Some Republicans are discounting the effort as a non-starter, but Democrats are zoning in on those on the other side that they believe can be brought on board, including Senator Susan Collins.
The moderate from Maine called the amendment "promising," but said she wanted to read the language carefully before committing her support.
Toomey, from Pennsylvania where many rural communities support gun rights, appeared to poke a hole in the argument that any fresh measures should be viewed as an infringement of the constitutional right to bear arms.
"I don't consider criminal background checks to be gun control," said Toomey, who like Manchin is a gun owner and hunter.
"It's just common sense. If you pass a criminal background check, you get to buy a gun."
Their amendment, aided by Republican Mark Kirk from Illinois, where the largest city Chicago is awash with gun violence, stops short of the universal background checks sought by Obama in the aftermath of Sandy Hook.
Specifically, it would not affect person-to-person sales, such as those between relatives, with Manchin assuring that "personal transfers are not touched whatsoever."
But it would strengthen existing law, which currently only requires background checks when guns are purchased from a licensed dealer.
Toomey also sought to ease worries that the background checks could lead to a national registry of gun owners, saying the proposed legislation "does not require record-keeping by any private citizens."
The Senate Democratic leadership has already introduced a three-part bill that would require expanding background checks to virtually all gun sales, make gun-trafficking a felony, and boost funding for school safety.
Majority Leader Harry Reid has scheduled a procedural vote Thursday for the overarching bill, to which the Manchin-Toomey amendment would be attached.
A group of 13 Senate Republicans vowed to obstruct the broader bill, but by late Tuesday that effort was weakening, with other Republicans signaling they would vote for moving ahead.
Republican backing for any gun legislation is crucial for the measures to pass Congress, especially the Republican-led House.
The National Rifle Association issued a statement on Wednesday saying that "expanding background checks at gun shows will not prevent the next shooting, will not solve violent crime and will not keep our kids safe in schools."
Expanding background checks enjoys sweeping public support, with polls showing about nine in 10 Americans in favor, and is seen as more viable legislation than an assault weapons ban and limits on high-capacity magazines.