Washington (CNN) -- In a breakthrough on gun legislation, two U.S. senators -- a Democrat and a Republican -- announced Wednesday they had worked out 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.
In an emotional scene later on Wednesday, Manchin choked up while meeting with relatives of Newtown victims who praised him for his political courage in taking on the powerful gun lobby.
"You give me more legislative strength than you know," Manchin said at one point.
He then was unable to speak when asked by a reporter how the Newtown families affected his role in the negotiations with Toomey and others.
Despite the agreement reached by Manchin and Toomey, both rated as strong supporters of gun rights by the influential National Rifle Association, the prospects for significant gun legislation to win congressional approval remained uncertain.
The NRA responded to the Manchin-Toomey agreement by saying it would fail to address the core issues of gun violence.
"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," it said in a statement.
Following the Newtown shootings by a lone gunman, Obama called for a series of proposals including "universal" background checks on all gun purchases. Currently, the federal law requiring background checks covers licensed firearms dealers, with private sales excluded.
Fierce opposition by the NRA and its allies in Congress -- mostly conservative Republicans but also some Democrats from gun-friendly states -- made clear that the universal checks sought by Obama had no chance of passing, leading to efforts by Manchin, Toomey and others to work out a compromise.
"The bottom line for me is this: If expanding background checks to include gun shows and Internet sales can reduce the likelihood of criminals and mentally ill people from getting guns and we can do it in a fashion that does not infringe on the Second Amendment rights of law-abiding citizens, then we should do it, and in this amendment I think we do," Toomey told reporters on Wednesday.
Asked later about criticism by the NRA and others, he told CNN that the proposed legislation was "not a cure-all, but I think it would be some progress."
Manchin noted that the proposal meant that firearms buyers at gun shows would face the same background check currently required in sales by federally licensed gun dealers. In addition, it would close a loophole that exempts intrastate gun sales on the Internet from requiring a background check, he said.
Addressing concerns of the NRA that expanding background checks would burden law-abiding gun owners seeking to trade or gift weapons in a personal transfer, Manchin declared that "personal transfers are not touched whatsoever."
Another provision would recognize the legitimacy of concealed weapons permits across state lines.
The Manchin-Toomey compromise also would require states and the federal government to provide records on criminals and the "violently mentally ill" to the national background check system, addressing a criticism by the NRA and other opponents of gun laws that the existing system lacks substantive information.
In addition, the plan calls for a new National Commission on Mass Violence to report in six months on "all aspects of the problem, including guns, school safety, mental health, and violent media or video games."
The NRA said rejection of the universal checks sought by Obama was "a positive development," and it called for "serious and meaningful solutions" to gun violence instead of "blaming law-abiding gun owners for the acts of psychopathic murderers."
Obama said there are aspects of the proposal that he would like to see strengthened.
"But the agreement does represent welcome and significant bipartisan progress. It recognizes that there are good people on both sides of this issue, and 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.
"Congress needs to finish the job," Obama added, saying he would continue "asking the American people to stand up and raise their voices because these measures deserve a vote."
Other reaction ranged from cautious support to angry rejection.
The Brady Campaign, named after the former White House press secretary wounded in an assassination attempt on President Ronald Reagan, called the compromise a "good step forward," while New York Gov. Andrew Cuomo described it as "better than nothing" but a sellout to the gun lobby.
"This is a Congress that is captive of the extremists and there is no clearer proof of that than this," Cuomo said on the "Capitol Pressroom" radio show, adding that the compromise meant "we are not talking about a significant package of gun control anymore."
Manchin told reporters the compromise on background checks would be the first amendment offered when the Senate begins debating the package of gun legislation.
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.
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.
House Speaker John Boehner, R-Ohio, was noncommittal on Wednesday on prospects for gun legislation, telling reporters he would wait to see what the Senate passes.
Meanwhile, Democratic Rep. Mike Thompson of California and Republican Rep. Peter King of New York said they planned to introduce legislation in the House similar to the Senate compromise by Manchin and Toomey.
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.
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.' "
Failing to pass new 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 in their corner 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 build 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.
"The one thing you know today is that if the government creates a record, it's not secure," Keene said, adding that requiring background checks on all gun sales -- the so-called universal system -- raised the question of "is it linked to a national registration scheme."
According to a summary of the compromise proposal, it includes language that prohibits creation of a national gun registry or misusing information from background checks.
The high political stakes of the divisive gun law debate have bred 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 Paul Courson, Ted Barrett, Paul Steinhauser and Ashley Killough contributed to this report.