A bipartisan pair of senators introduced a compromise proposal to expand background checks on Wednesday, an agreement which could form the basis for major gun control legislation to potentially pass through Congress.

Sens. Pat Toomey, R-Pa., and Joe Manchin, D-W.Va. – who both enjoy top ratings from the National Rifle Association – outlined an agreement that would expand background checks to include most firearms sales, including those at gun shows and online.

Related: Background checks for guns - What you need to know

While the new framework does not go quite as far as the stricter gun controls first advocated by President Barack Obama in the wake of December’s Newtown, Conn., shootings, it paves the path for Senate approval of one of the president’s major second term initiatives.

“Truly, the events of Newtown changed us all,” Manchin said at a press conference announcing the agreement. “This amendment won't ease the pain ... but nobody here – and I mean not one of us in this great Capitol of ours – can sit by and not try to prevent a day like that from happening again.”

Related: NRA a constant presence in background check deal

The agreement won the support of a key proponent of new gun legislation, New York Sen. Charles Schumer, the No. 2 Senate Democrat who said he planned to co-sponsor the new agreement. Schumer called Vice President Joe Biden, New York City Mayor Michael Bloomberg, Mark Kelly (the husband of former Rep. Gabrielle Giffords, D-Ariz.) and various gun safety groups to urge them to support the compromise, as well.

In a statement issued later Wednesday afternoon, Obama said that he applauded the agreement. 

"This is not my bill, and there are aspects of the agreement that I might prefer to be stronger," the president said. "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."

The background check deal made several tweaks to the prior Democratic proposal, namely by striking a provision requiring states to recognize concealed carry permits from other states, and eliminating another measure exempting sellers who sell five guns per year or fewer from the background check requirement.

J. Scott Applewhite / AP

Sen. Joe Manchin of West Virginia, D-W.Va., left, and Sen. Patrick Toomey, R-Pa., arrive at a news conference on Capitol Hill in Washington, Wednesday, April 10, 2013, to announce that they have reached a bipartisan deal on expanding background checks to more gun buyers.

The proposal also includes other provisions meant to allay gun-rights advocates' concerns about background checks. Namely, the legislation would not require background checks for intra-family transfers of firearms, and would apply existing record-keeping rules used for gun stores to those weapons sold online or at gun shows.

But the NRA quickly criticized the new proposal as inadequate, a pronouncement which could influence the decision-making of wavering lawmakers.

“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,” said the gun rights group. “The sad truth is that no background check would have prevented the tragedies in Newtown, Aurora or Tucson.”

(Democratic aides still think the NRA might not necessarily throw its full weight behind opposing the proposal, though, especially because the gun rights group's representatives were a near-constant presence during the Manchin-Toomey talks.)

Still, the bipartisan nature of the agreement could improve prospects for its approval by the entire Senate, especially if the Obama administration should throw its weight behind the proposal. Support from Toomey (as well as Illinois Sen. Mark Kirk, another swing-state Republican) could entice other GOP senators to support the compromise.

“I’m a gun owner, and the rights that are enshrined in the Second Amendment are very, very important to me personally,” Toomey said alongside Manchin on Capitol Hill. “But I’ve got to tell you, candidly, that I don’t consider criminal background checks to be gun control. I think it’s just common sense.”

The first test of that support will come on Thursday, when Senate Majority Leader Harry Reid, D-Nev., said he planned to hold a key vote to move forward with the gun debate. A group of conservative senators – including Senate Minority Leader Mitch McConnell, R-Ky. – had vowed to filibuster any gun legislation, though their ability to wage one successfully was undercut by several other Republicans, who said they would not support such a maneuver.

Already, groups on opposite sides of the gun debate have aligned for or against the Manchin-Toomey proposal.

Americans for Responsible Solutions, the group founded by Kelly to support gun control, said it was “pleased” by the new agreement.

“We will do everything in our power to ensure that Americans know about the determined leadership of Sen. Manchin, a conservative Democrat, and Senator Toomey ... to keep this common sense legislation moving,” said Pia Carusone, the group’s executive director.

But the conservative Heritage Action also issued a statement on opposition to the gun deal, a declaration that could weigh heavily on Republicans in the House, where any Senate legislation awaits an uncertain future.

House Speaker John Boehner, R-Ohio, talking on Wednesday about the forthcoming Senate accord, was noncommittal about bringing up a prospective gun bill for a vote.

"As I've made clear, any bill that passes the Senate, we're going to review it. In the meantime, we're going to continue to have hearings looking at the source of violence in our country," Boehner said at a press conference. "It's one thing for two members to come to some agreement; it doesn't substitute the will of the other 98 members."

Raising hope, though, for House support was another bipartisan pairing, Reps. Mike Thompson, D-Calif., and Peter King, R-N.Y., who hailed the Manchin-Toomey agreement, and said they planned to work to introduce similar legislation in the lower chamber. 

"This legislation is enforceable, it will save lives, and it respects the Second Amendment rights of law abiding Americans," the lawmakers said in a joint statement.

This story was originally published on Wed Apr 10, 2013 11:17 AM EDT

Senators negotiating a bill mandating background checks for all gun buyers are privately expecting the National Rifle Association not to fight the measure -- provided the legislation does not require private gun sellers to maintain records of the checks, NBC News has learned.

If that requirement is met and key Republican negotiator Sen. Tom Coburn of Oklahoma signs on, the powerful gun lobby has signaled to lawmakers that they would not actively oppose the bill -- and not count votes in favor of it as part of its highly influential NRA lawmaker ratings -- according to Senate aides familiar with the stalled negotiations.

Such a deal could clear the way for a universal background check bill, a central tenet of President Barack Obama's gun control initiative, to pass the Senate with significant Republican support. Odds of passage in the House would brighten significantly as a result.

The NRA denies being part of any agreement. "We do not take positions on hypotheticals. We will make our position known if and when legislation is introduced," said Chris Cox, the group's top lobbyist. 

The NRA is still adamantly opposed to expanding background checks to private sales. "To be clear, the National Rifle Association does not support legislation that would criminalize otherwise lawful transfers of firearms between law abiding Americans," Cox said.

EARLIER: Panel advances background check bill, but its path remains clear

Such a decision would mark the first major compromise from the group in the wake of the elementary school shootings that killed 26 in Newtown, Conn. The NRA has signed on to a bipartisan mental health bill and are talking to senators about gun trafficking legislation. But with an assault weapons ban not likely to pass, the background check bill is the main legislative fight over new gun restrictions.

Still, Democrats have balked at this development: Leaving such a provision out, they say, would make the law toothless and would never earn support from gun control advocacy groups.

But without the record-keeping provision -- and the blind eye from the NRA -- Democrats would instead have to squeak their way to the 60 votes needed to pass in the upper chamber, potentially risking the whole package.

Both sides say it's tough to compromise on the issue of records. Coburn and the NRA argue that requiring private sellers to keep a record of each background check and gun sale would start down a slippery slope to a national gun registry. Gun control advocates, on the other hand, say enforcing any background check law would be all but impossible if records aren’t kept, and thus would have little impact on the criminals the bill is designed to target.

Jacquelyn Martin / Jacquelyn Martin / AP

Sen. Tom Coburn, R-Okla. walks toward the Senate chamber on Capitol Hill in Washington, Monday, Dec. 31, 2012.

"With about 90 percent of the public calling for a comprehensive background check system, the incentive to agree to a bill that only law abiding people will comply with, and that people we're trying to reach will ignore, is vanishingly small," said Mark Glaze, the executive director of Mayors Against Illegal Guns, a group headed by New York City Mayor Michael Bloomberg.

"Background checks do not and cannot lead to gun registration. It's a lie meant to muddy the debate and distract from our common goal -- saving lives with solutions that Americans support overwhelmingly," Mark Kelly, the husband of former Rep. Gabby Giffords, said in a Tuesday statement.

Coburn, who holds an "A" rating from the NRA, has been locked in closed meetings with Sens. Chuck Schumer, D-N.Y., and Joe Manchin, D-W.Va., over the background check legislation.

Coburn and Manchin insist they're still talking, and Manchin said he was on the phone Tuesday with the NRA. He and Coburn have shared legislative language with the group as they've worked on an agreement.

"Tom Coburn and I come from a culture, a background, a culture of guns. So I think the NRA would respect and appreciate where we're coming from, with the understanding that we understand our NRA members," Manchin told NBC News on Tuesday. "I'm an NRA member, Tom's an NRA member, and they're looking at that. And hopefully we can find some common ground."

But the background check discussions with Schumer have stalled -- and the New Yorker is now reaching out in an effort to find another Republican who might be willing to sign on. The list includes moderates, like Sen. Susan Collins of Maine, and Republican senators who aren't running for re-election, like Georgia Sen. Saxby Chambliss.

Chip Somodevilla / Getty Images

Sen. Chuck Schumer boards an elevator at the U.S. Capitol on March 12, 2013 in Washington, D.C.

Sources familiar with the negotiations suggest that Schumer has only made a public break with Coburn because he's trying to convince groups on the left -- the Bloomberg group, the Brady Campaign to Prevent Gun Violence, and others -- that they need to give in on record-keeping in order to save a broader package.

Glaze said that's a no-go, indicating that his group -- backed by Bloomberg's billions -- would instead air ads in senators' home states attacking them for not going further in their quest to stop gun violence.

"We're going to take the time to get out into the country and have that conversation with members and their constituents, and we will see where we end up in the late summer," Glaze said.

Asked about the groups' position Tuesday, Manchin emphasized the realities of getting a bill passed.

"I respect their position, but basically, we have 535 members, and they all have certain concerns and considerations to be taken. If you want to pass something, you have to work within a, a compromise if you will, bipartisan arena," he said.

NBC's Mike Viqueira contributed. 

This story was originally published on Tue Mar 12, 2013 5:01 PM EDT