WASHINGTON (CNN) -- Two U.S. senators, a Democrat and a Republican, announced on Wednesday they had reached a compromise on expanding background checks on firearms buyers to include gun shows and Internet sales.
The deal reached by Sens. Joe Manchin, D-West Virginia, and Pat Toomey, R-Pennsylvania, sets up the likelihood of a major Senate debate on gun legislation starting as soon as Thursday, when the chamber is expected to overcome a GOP filibuster attempt to block the proposals.
President Barack Obama and leading Democrats have pushed for tighter gun laws in the aftermath of the December school massacre in Newtown, Connecticut, that killed 20 first-graders and six educators.
Manchin and Toomey are both rated as strong supporters of gun rights by the influential National Rifle Association.
Currently, the federal law requiring background checks covers licensed firearms dealers, with private sales excluded.
A Democratic leadership source said the compromise will probably be the first amendment offered to a package of gun laws passed by the Senate Judiciary Committee.
Senate Majority Leader Harry Reid, D-Nevada, said Tuesday he would hold a vote on opening debate on the gun package Thursday, putting pressure on Manchin and Toomey to finalize their agreement intended to overcome a Republican filibuster of the legislation.
The filibuster pledged by 14 GOP senators means Reid, whose Democratic caucus holds 55 seats, needs 60 votes to open debate on the gun legislation.
Democrats believe that as many as a dozen GOP senators will vote with them, making up for the handful of pro-gun Democrats who might vote against launching debate on the bill.
"The way you put together a coalition to pass the bill is to allow as many amendment votes as you can. We are willing to take the time to do that and have that process," the Democratic leadership aide said.
Obama has made gun measures a major focus of his second-term agenda, holding events across the country to push for Congress to vote on the package.
He spoke Monday in Connecticut, where the Newtown shootings occurred, and Vice President Joe Biden made a similar call for action at the White House on Tuesday.
A successful GOP filibuster would prevent a vote on specific components of the legislative package. Even if an amended bill passes the Senate, approval from the Republican-led House remains uncertain.
Obama's rhetoric has reflected the political uncertainty, with the president and his aides using increasingly personal language intended to shame Republicans into allowing public votes on measures that have public support but are fiercely opposed by the NRA.
"If senators don't have the guts to go on the record to vote how they feel on this issue ... that would be a shame and that would be a disservice to their constituents," White House spokesman Jay Carney told reporters Tuesday.
At a later White House event intended to keep up the public profile of the issue, Biden said Republican efforts to block tougher gun laws showed they were in a "time warp" because public support on issues such as expanded background checks "has moved beyond where it was five, 10, even three years ago."
On the other side, the NRA and its supporters in Congress say the Democratic proposals threaten the constitutional right to bear arms, and also offer ineffective responses intended as political show instead of real solutions to the problem of gun violence in America.
"On firearms questions, on Second Amendment questions, there's a divide in this country," NRA President David Keene told CNN. "To call it an ideological divide is too simple because it's a cultural divide. When something happens, the folks on the other side from us say, 'well the problem's the gun, we need to do something about guns.' "
Defeat of any package of gun laws would be a stinging defeat for Obama and Democrats.
However, a public perception that Republicans blocked popular proposals such as expanding background checks could harm GOP prospects in 2014 and 2016 among moderates they need to have any chance of countering strong support for Democrats by minority demographics such as Hispanic Americans, African Americans and the gay-lesbian vote.
A new national survey showed that 86% of Americans support some expansion of background checks.
At the same time, the CNN/ORC International poll released Wednesday also showed a majority of respondents fear that increased background checks would lead to a federal registry of gun owners that could allow the government to take away legally owned weapons.
Keene and other opponents worry that an expanded background check system would create a paper trail that could eventually be used to create a national gun registry, which they reject as unconstitutional.
They also contend it would prove a burden to law-abiding gun owners while doing nothing to stop criminals from getting hold of firearms.
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However, Biden accused the NRA of spreading false information that the proposed legislation would intrude on Second Amendment rights.
The high political stakes of the divisive gun law debate breed hardball tactics and strategies. The NRA has long kept a comprehensive scorecard of the voting records of legislators on gun issues, which it combines with campaign contributions to try to influence elections.
In response, a group led by New York Mayor Michael Bloomberg called Mayors Against Illegal Guns announced this week it was launching its own scorecard to identify members of Congress who vote against tougher gun laws.
The Senate Judiciary Committee passed a package of gun laws proposed by Obama after the Newtown attack by a lone gunman.
Proposals in the committee's package included expanding background checks on gun buyers, toughening laws against gun trafficking and straw purchases, banning semiautomatic rifles modeled after military assault weapons as well as large-capacity ammunition magazines, and coming up with ideas for improving school safety.
The weapons ban, which would update a similar 1994 law that expired a decade later, already has been dropped, though Reid has promised a floor vote on it as an amendment to the package.
Some states already have passed stricter gun laws similar to the federal proposals since the Newtown shootings. They include Connecticut, where the killings occurred, and Colorado, the site of two other notorious mass shootings that contributed to a renewed gun debate in America.
The current background check system was created in 1989. It requires federally approved gun dealers to check whether gun buyers have a criminal background or other problem to make them ineligible to purchase a firearm.
Under the system, the gun dealer maintains a record of the transaction, but the federal government keeps no such identifying paperwork.
According to a Justice Department report, less than 2% of those seeking to purchase firearms were denied because of background checks from 1998 through 2009.
Opponents cite that figure as evidence that the system fails to stop illegal weapons sales that the legislation seeks to target, while supporters say the result shows the system keeps some guns out of the hands of the wrong people and the system should be expanded and strengthened.
CNN's Ted Barrett, Paul Steinhauser and Ashley Killough contributed to this report.