WASHINGTON — A bipartisan collection of senators on Wednesday announced a compromise measure to expand background checks for gun buyers, increasing the chances that a viable package of new gun safety laws will soon hit the Senate floor.
Senators Joe Manchin III, Democrat of West Virginia, and Patrick J. Toomey, Republican of Pennsylvania, have spearheaded the deal.
Under the terms of the agreement, background checks for gun buyers would expand to gun shows and online sales — a huge portion of gun sales that are made without the background checks used by gun stores — and will maintain record-keeping provisions that law enforcement officials find essential in tracking criminal gun use, but that gun rights groups find anathema.
The bill would also modify the current measure, which will receive a procedural vote on Thursday, in other key ways. For example, the background checks provision would extend to fewer gun buyers than many gun control groups — and some Democrats in the Senate — would like. Sales between family members, for instance, would be exempt.
Further, other provisions of the current background component of the gun safety bill were removed, including one requiring that each state validate another state’s concealed carry permits and another one limiting Internet sales to five or more guns per year.
The compromise, which two weeks ago seemed elusive, is designed to pull in as many members from both parties as possible — including Democrats running for re-election in Republican-leaning states — to favor the bill.
“This is a start,” Mr. Manchin said. “This is not the end of our work.” He added: “Americans on both sides of the debate can find common ground. I can’t say enough about my friend Pat Toomey.”
Mr. Toomey said gun legislation was “not something I sought,” but that he felt it was inevitable and that he wanted to contribute to something he could support.
“There was the danger that we might not accomplish anything,” Mr. Toomey said, adding that he and Mr. Manchin have consulted with the National Rifle Association.
Mr. Toomey said, “I’ve got to tell you, candidly, I don’t consider criminal background checks gun control.”
The agreement contains changes that will draw the attention of gun rights supporters and gun control advocates. The original measure would have allowed someone to have weapons that would be prohibited in a state by claiming they were just passing through; under the agreement, the burden would shift to the traveler to demonstrate that he or she was actually traveling, and the burden would not be on the government to prove it.
But the new agreement has other provisions that are in the current proposal headed for the floor. Gun sellers would be given immunity from lawsuits if a firearm that they sell is used in a crime. It would also permit dealers to travel to another state and transfer guns at a gun show in the other state as long as they abide by the laws of the other state. And it would permit active-duty military members to buy guns in their home states and in states where they are based.
Senator Christopher Murphy, a freshman Democrat from Connecticut, chose to focus his first speech on the Senate floor on gun violence Wednesday morning, bringing large photos of some of the 20 children killed in the Newtown massacre to the floor with him. “I believe that we’re good enough to drown out the voices of the status quo and the lobbyists and political consultants,” Mr. Murphy said.
The National Rifle Association, which has long supported Mr. Manchin and Mr. Toomey, released a statement after the morning news conference to denounce the proposed amendment.
“The sad truth is,” the statement read, in part, “that no background check would have prevented the tragedies in Newtown, Aurora or Tucson. We need a serious and meaningful solution that addresses crime in cities like Chicago, addresses mental health deficiencies, while at the same time protecting the rights of those of us who are not a danger to anyone.”
Before details of the plan were announced on Wednesday morning, Senator Charles E. Schumer, Democrat of New York, made calls to Vice President Joseph R. Biden Jr., who is overseeing the issue for the White House; Mayor Michael R. Bloomberg of New York, who has made gun control one of his signature issues; and Mark Kelly, the husband of former Representative Gabrielle Giffords, who was severely injured in a mass shooting, to inform them of the details and ask for their blessings.
Ms. Giffords’s group, Americans for Responsible Solutions, later announced support for the agreement.
“With enactment of this proposal, criminals and the dangerously mentally ill will no longer be able to get weapons at gun shows and in the shadows of the Internet without passing the kind of sensible background check that responsible gun owners undergo,” said Pia Carusone, the group’s executive director. “Gun violence isn’t a partisan issue, and we hope that all senators listen to the over 90 percent of Americans who know it will have a measurable effect on keeping our communities safer and want to see this legislation passed.”