The US Senate faces a showdown Wednesday over gun rights, but even supporters of a contentious compromise on background checks conceded they might still be short of the necessary votes.
Majority Leader Harry Reid said he scheduled up to nine votes on amendments, including one that expands background checks to all commercial sales, and others that ban assault weapons, restrict the size of ammunition clips and criminalize gun trafficking.
Republican substitute amendments that largely boost gun rights, including one that addresses the concealed-carry rights of gun owners, are also among those to be voted on.
Reid struck a deal with top Republican Mitch McConnell to hold the votes beginning at 4:00 pm (2000 GMT), on the most significant US gun bill in nearly 20 years, but it remained unclear late Tuesday whether he had enough support for the background check measure, the bill's core amendment.
"We're still working on it, we don't have a final count," number two Democrat Dick Durbin told reporters.
President Barack Obama voiced optimism about the proposed bill and said it would be "unimaginable" if Congress defied strong public support for the measure, especially after the Newtown school massacre in December.
Reid insisted they had the momentum to pass the legislation, which would expand background checks to all commercial firearms sales, including at gun shows and on the Internet.
Gabrielle Giffords, the former congresswoman who was shot in the head in 2011 and has emerged as a powerful voice for stronger gun laws, made a dramatic appearance on Capitol Hill to lean on some undecided members.
"I'm optimistic, I think we can get there," her husband Mark Kelly, a retired NASA astronaut, told AFP as he and Giffords, who both strongly back the constitutional right to bear arms, began their last-minute appeals.
But the numbers were not there on Tuesday, one of the architects of the compromise conceded.
"We're not ready for a vote," Senator Mark Kirk, who helped craft the legislation with fellow Republican Pat Toomey and Democrats Joe Manchin and Chuck Schumer, told reporters after debate on the measure officially began.
Supporters would need 60 votes in the 100-member chamber to overcome blocking tactics and ensure final passage. Democrats and independent allies hold 55 seats.
But with a handful of Democrats including Mark Begich of Alaska facing tough 2014 re-election fights in red-leaning states, Durbin said his side would need "nine or 10" Republicans on board to meet the threshold.
"They don't have the votes to pass it... and I think they know it," Iowa Republican Chuck Grassley told the Senate.
Schumer said his side was trying to round up votes from moderate Republicans such as John McCain and a handful of Democrats, but such lobbying was being countered by that of the powerful National Rifle Association (NRA).
Schumer and Manchin acknowledged they were considering allowing tweaks to their amendment, including one that could exempt gun buyers in remote rural areas from needing to obtain a federal firearm license from a gun dealer.
"It has possibilities, if it will win us some votes," Schumer said.
Last week the Senate agreed to debate what has emerged as the most significant gun legislation in nearly 20 years -- 16 Republicans voted to proceed to the bill, arguing it deserved a debate and a vote, but several of them have said they would not support it.
The deal got a boost when the second-largest US gun rights group, the Citizens Committee for the Right to Keep and Bear Arms, came out in favor of it.
In addition to background checks, the legislation would stiffen penalties for gun trafficking and boost school safety measures.
Some Republican amendments being put forward are seen by gun control advocates as watering down the legislation.
With the bill under threat, Obama has engaged in a full-court press, phoning Republicans as well as Democrats from right-leaning states.
He met with Schumer and McCain Tuesday to discuss a different subject -- the bipartisan immigration reform plan which Schumer acknowledged might have a better chance of seeing the light of day.
"There are many in the conservative movement who want to see an immigration bill done," Schumer said.
"There are very few in the conservative movement who want to see a gun bill done."