US gun control advocates are making an emotional push for new restrictions as the Senate weighs whether to debate a law expanding checks on gun buyers.
On Thursday, religious leaders presided over a vigil in Washington DC at a mock cemetery for more than 3,300 people killed in gun violence since December.
Meanwhile, activists are reading aloud the names of shooting victims.
If the Senate votes to begin debate on the background checks bill, it will mark the first such move in years.
Gun rights advocates have vowed to oppose new restrictions, saying the US Constitution forbids Congress from restricting gun sales.
The lobbying push by both gun control and gun rights groups comes as a Democratic and a Republican senator have announced an agreement on a bill to expand background checks.
On Wednesday, Democrat Joe Manchin and Republican Patrick Toomey unveiled a deal that would expand checks to all online and all gun show sales, establish a commission on mass violence and ease some restrictions on transporting guns across state lines.
Currently, so-called private gun sales by dealers who are not licensed, including some at gun shows, are not subject to criminal background checks on the purchaser.
The Senate's Democratic leadership believes it now has enough Republican support to thwart a conservative attempt to stop gun control legislation even being considered on the floor of the chamber.
If the motion to begin debate passes the 60-vote threshold on Thursday, it will be the furthest in the legislative process any gun control bill has moved since 1994, when an assault weapons ban passed.
But analysts caution the bill must clear several hurdles before it becomes law.
Vice President Joe Biden, a strong supporter of new gun control legislation, told MSNBC's Morning Joe programme on Thursday that gun legislation was "one of the cases where the public is so far ahead of the elected officials".
Mr Biden also accused the nation's top gun rights lobby, the National Rifle Association (NRA) of spreading disinformation, and promised expanded background checks would not lead to a national gun registry.
Bipartisan deal
But the NRA opposes the Manchin-Toomey deal, arguing background checks do nothing to prevent gun violence.
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In a letter to senators on Wednesday, NRA lobbyist Chris Cox warned that the organization would score lawmakers based on their votes on the Manchin-Toomey deal and other amendments it opposes.
President Barack Obama's other proposals, including a ban on assault weapons and high-capacity ammunition magazines, have not gained traction in Congress.
In a statement Mr Obama said he wished the agreement was stronger but praised the move as progress.
"We don't have to agree on everything to know that we've got to do something to stem the tide of gun violence," he said.
If Thursday's vote clears the procedural hurdle, Senators would then vote on a series of amendments to the legislation and then once more to close debate, before voting on the bill itself.
Prospects for legislation in the House are unclear, with Republican House Speaker John Boehner saying he will wait until the Senate has a definite outcome before deciding whether to bring the bill to the House floor.
"It's one thing for two members to come to some agreement," he said. "It doesn't substitute the will for the other 98 members."
- BBC
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