The most significant US Senate debate on gun laws in 20 years starts Tuesday with the chamber introducing a bipartisan compromise on background checks, as both sides expect a razor-thin vote this week.
The measure's two primary authors, Democrat Joe Manchin and Republican Pat Toomey, spent much of Monday scouring the 100-member chamber for last-minute support for the deal, which got a surprise boost when the second-largest US gun rights group came out in favor of it.
There was no firm sense late Monday that the amendment's proponents, including Senate Democratic leaders, had the 60 votes necessary to ensure final passage, "but we're working on it," Toomey told AFP.
A day earlier the Pennsylvania Republican said it remained "an open question" as to whether or not he would have sufficient backing.
"I've talked to everybody," Manchin told reporters after trying to sell the measure's merits in the Senate. "It's very close."
The underlying bill would expand background checks to all commercial firearm sales, including those at gun shows and on the Internet. It would also stiffen penalties for gun trafficking and boost school safety measures.
Several amendments are expected to be put forward, including some which gun control advocates say would water down the legislation.
With the bill under threat, President Barack Obama has engaged in a full-court press, phoning Republicans as well as Democrats from right-leaning states.
And Gabrielle Giffords, the former congresswoman who was shot in the head in 2011 and who has emerged as an advocate for commonsense measures to boost gun safety, will be on Capitol Hill this week seeking to convince senators to vote in favor of expanded background checks.
The amendment recently received the endorsement from the Citizens Committee for the Right to Keep and Bear Arms, the second largest pro-gun group after the National Rifle Association, which has come out against expanding the checks.
"If you read the Manchin-Toomey substitute you can see all the advances for our cause that it contains," wrote Citizens Committee chairman Alan Gottlieb, who highlighted the measure's plan to allow interstate sales of handguns and specifically forbid the creation of a gun registry.
Despite such nods to pro-gun lawmakers, the bill faces an uncertain fate.
Sixteen Republicans voted last week to begin debate on the bill, but several of them, as well as some moderate Democrats facing tough election fights in 2014, have said publicly they are not likely to vote for it.
Focus has turned to the few lawmakers whose positions were not yet carved in stone. They include Democrat Heidi Heitkamp, whose office confirmed that she remains undecided, and Republican John McCain, who hinted Sunday that he could support the bill.
The only Senate Republicans officially on board are Toomey, co-author Mark Kirk, and moderate Susan Collins.
"I've been working it," Kirk told AFP.
Manchin and Toomey will likely need at least seven Republicans to vote in favor, Kirk said, adding that "it's a pretty high hurdle for 60" votes.
Democrat Patrick Leahy, the Senate's longest-serving member and a fierce protector of gun rights, took to the floor to say: "No one is going to take away our Second Amendment rights. They're not at risk."
The smaller, Independent Firearms Owners Association agreed, coming out in favor of expanding background checks on Monday.
Director Richard Feldman insisted there was no schism between his group and the NRA, but said that "we're not in lockstep."
And yet he acknowledged that those pressing for the legislation faced a tough climb.
"There are multiple hurdles before it's going anywhere," he said.