Molly Riley/UPI/Landov

ABC News' Jeff Zeleny and Sunlen Miller report:

Sens. Joe Manchin and Pat Toomey announced a bipartisan compromise today seeking to expand background checks for the sale of guns - both at gun shows and online - that could help break the impasse in the gun-control debate in Congress.

The deal would be added as an amendment to existing gun legislation in the Senate. It opens the door to the biggest potential change in U.S. gun laws in nearly two decades, but still faces deep questions and lingering opposition when the Senate begins voting Thursday on whether to proceed with the gun debate.

The compromise is not expected to be debated until next week.

"Today is the start of a healthy debate that must end with the Senate and House, hopefully, passing these commonsense measures and the president signing it into law," Manchin, a West Virginia Democrat, said.

"The event of Newtown, truly the events at Newton, changed us all. It changes our country, our communities, our town and it changed our hearts and minds."

The agreement is intended to make the legislation on guns more palatable to Democratic and Republican lawmakers, who have voiced skepticism about some provisions. The proposal calls for exempting background checks for guns sold among between family members, which was a key sticking point for many supporters of gun rights, including hunters and sportsmen.

When asked by ABC News whether he was worried about risking his "A" ranking with the National Rifle Association, Toomey said: "What matters to me is doing the right thing. And I think it's the right thing."

The agreement was also forged with Sens. Chuck Schumer, D-N.Y., Mark Kirk, an Illinois Republican. Toomey, a Republican from Pennsylvania, said today he was uncertain whether any other Republicans would support the bill.

"I've got to tell you candidly, I don't consider background checks to be gun control. I think it's just commonsense," Toomey said. "If you pass a criminal background check, you get to buy a gun, no problem. It's the people who fail a criminal or a mental health background check that we don't want to have a gun."

The name of the agreement underscores its goal to try to appeal to both sides of the gun debate: The Public Safety and Second Amendment Rights Protection Act. The bill would not require record-keeping of guns for private citizens, which is a key sticking point for critics.

The NRA was advised of the compromise and has been a part of the discussions. It remained an open question how forcefully the group would oppose the measure. A statement from the organization called it "a positive development" that the universal background checks that were part of President Obama's gun bill would be replaced.

In an interview with ABC News, Manchin that he did not believe his compromise on background checks should be opposed by the NRA. But he said that it was worth risking a lower grade from the NRA.

"We're here for a purpose to do things. This is the right thing to do," Manchin said. "I would hope that just having a score or a rating they would look at it and say, 'Did we do the right thing? Did we use common sense? Did we do the right thing to protect lives, protect our children?' I would hope we would get credit for that. I can't help the scoring system."

He added: "I would think the NRA would have to say these are good things from sensible, reasonable gun owners."

Several advocates of stronger gun control measures announced their support for the plan, including Americans for Responsible Solutions, which is the organization led by former Rep. Gabrielle Giffords, the victim of an attempted assassination in 2011.

"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," the group said.

"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."