Sens. Joe Manchin, D-W.Va., and Patrick Toomey, R-Pa., said their accord would help keep firearms from criminals and the mentally ill. Subjecting more firearms purchases to federal background checks has been the chief goal of Obama and gun control supporters, who promote the system as a way to prevent criminals and other potentially dangerous people from getting the weapons.
The agreement between two of the most conservative members of each party was expected to make it even likelier that the Senate's initial vote Thursday to begin debating gun legislation will succeed, despite an effort by conservatives to block consideration of the measure.
Even so, the ultimate fate of gun legislation remains unclear, clouded by opposition from the National Rifle Association and many Republicans and moderate Democrats in the Democratic-led Senate and the Republican-run House. Many critics say the effort would violate the Second Amendment right to bear arms and burden law-abiding gun owners.
"Truly the events at Newtown changed us all," said Manchin, referring to the Connecticut town where 20 first-graders and six educators were shot down in December, launching the country into renewed debate over gun violence. "Americans on both sides of the debate can and must find common ground."
"I don't consider criminal background checks to be gun control," said Toomey. "I think it's just common sense."
In a written statement, the NRA was critical.
"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," the statement said. It said policymakers should focus on fixing the country's mental health system and on gang violence.
"President Obama should be as committed to dealing with the gang problem that is tormenting honest people in his hometown as he is to blaming law-abiding gun owners for the acts of psychopathic murderers," the NRA said.
The White House issued a written statement from Obama, saying he preferred stronger steps but still hailing the deal as 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 president said.
The administration was continuing its effort to pressure Congress on gun control on Wednesday as first lady Michelle Obama visited Chicago, the Obamas' hometown, where authorities say 29 current or former students have been shot in the past year.
Currently, the background check system covers sales only by licensed gun dealers. The compromise would apply the system to all commercial sales, such as transactions at gun shows and online. The sales would have to be channeled through licensed firearms dealers, who would have to keep records of the transactions.
Private transactions that are not for profit, such as those between relatives, would be exempt from background checks.
Sen. Chuck Schumer, D-N.Y., who led an earlier unsuccessful effort to strike a bipartisan background check deal, is backing the compromise after changes were made from an initial version of the deal between Manchin and Toomey, according to a Senate aide who spoke on condition of anonymity to describe details of the talks.
The changes included eliminating language that would have required states to recognize permits to carry concealed weapons issued by other states and eliminating language that would have limited background checks to sellers who sell at least five guns annually, said the aide.
Some Republicans might vote to begin debate on the legislation but eventually oppose the measure on final passage. Other parts of Obama's gun effort already seem likely to face defeat, including proposed bans on assault weapons and high-capacity ammunition magazines.
The gun legislation Reid wants the Senate to debate would extend the background check requirement to nearly all gun sales. Reid would try to replace that language with the Manchin-Toomey compromise once debate begins, a move that would require a vote.
The overall gun bill also tightens federal laws against illegal gun sales and slightly increases federal aid for school safety.
Thirteen conservatives have signed a letter saying they will block consideration of the measure, and Senate Minority Leader Mitch McConnell, R-Ky., said he will back that move. That will force Democrats to round up 60 votes to overcome the conservatives.
At least eight Republicans have said they want to begin debate or have indicated a willingness to consider it, a number that would be expected to grow if the background check agreement proves successful.
Some moderate Democrats were remaining noncommittal and could oppose opening the gun debate. There are 53 Senate Democrats and two independents who lean Democratic.
Amid the maneuvering, relatives of some Newtown victims are lobbying to support gun curbs. And Obama has been calling senators from both parties to push for the gun bill.
"People should listen to what we have to say and move the debate forward," said Mark Barden, who lost his 7-year-old son, Daniel. "It's not just about our tragedy. Lots of kids are killed every day in this nation. We have to help lead the change."___
Associated Press writers Nedra Pickler, Jim Abrams, Andrew Miga and Henry C. Jackson contributed to this report.